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1

Spectroscopy and reactions of molecules important in chemical evolution  

NASA Technical Reports Server (NTRS)

The research includes: (1) hot hydrogen atom reactions in terms of the nature of products produced, mechanism of the reactions and the implication and application of such reactions for molecules existing in interstellar clouds, in planetary atmospheres, and in chemical evolution; (2) photochemical reactions that can lead to molecules important in chemical evolution, interstellar clouds and as constituents in planetary atmospheres; and (3) spectroscopic and theoretical properties of biomolecules and their precursors and where possible, use these to understand their photochemical behavior.

Becker, R. S.

1974-01-01

2

Chemical Reactions  

NSDL National Science Digital Library

We don't often stop to think about it, but underlying many of our everyday activities are chemical reactions. From the cooking of an egg to the growth of a child, chemical reactions make things happen. Although many of the reactions that support our lives

National Science Teachers Association (NSTA)

2009-05-01

3

Chemical Reactions  

NSDL National Science Digital Library

We are going go over a general view of reactions to prepare us for our unit on Chemical Reactions! Have fun learning! WARNING: If you are caught looking at ANY other site, without permission, you will be sent to the ALC, and you will not participate in any other computer activities for the rest of the year. Get your worksheet and begin! Overview Take this quiz and have me come over and sign off on your worksheet when you have completed the quiz! Overview Quiz Next let's take a look at what effect the rate of a chemical reaction. Rates of Reactions Another quiz, another check off by me! Rates of Reactions Quiz Now how do we measure how fast a ...

Mrs. Hicken

2009-05-04

4

Studies of Elementary Reactions of Chemical Importance in the Atmospheres of Planets  

NASA Technical Reports Server (NTRS)

The paper discusses the following: 1. F + Cl2 Kinetics. Absolute rate constant for the reaction F(P-2) with Cl2 has been measured using the discharge flow kinetics technique coupled to mass spectrometric detection at T = 180 - 360 K and 1 Torr He nominal pressure. 2. Vapor pressure system. The main effort on the vapor pressure system involved the design and construction of an insulated enclosure ("Bakeout Box") to improve the uniformity of heating during the bakeout process. 3. Sunphotometer System. This period saw the completion of the two-channel sunphotometer, its calibration, and two field deployments. 4. Vibrational-to-translation (V-T) transfer rates for light hydrocarbons at low temperatures are important parameters in thermal-structure models of the upper atmospheres of the outer planets and their satellites. However, the required data are either simply not available or do not extend to the low temperatures found in those systems. Because methane is such an important constituent in outer planet atmospheres, we have initiated a program to measure the temperature dependence of (V-T) rates for its relaxation by appropriate collision partners. 5. The central focus of this research has been the vapor phase nucleation and growth of metals/refractory species into small particles and the aggregation of these primary particles into larger structures. These topics are part of the broader goal of understanding the conditions under which interstellar dust grains condense from stellar outflows and how these small dust grains coagulate into larger bodies such as planetesimals or planets.

Nesbitt, Fred L.

2003-01-01

5

Microfluidic chemical reaction circuits  

DOEpatents

New microfluidic devices, useful for carrying out chemical reactions, are provided. The devices are adapted for on-chip solvent exchange, chemical processes requiring multiple chemical reactions, and rapid concentration of reagents.

Lee, Chung-cheng (Irvine, CA); Sui, Guodong (Los Angeles, CA); Elizarov, Arkadij (Valley Village, CA); Kolb, Hartmuth C. (Playa del Rey, CA); Huang, Jiang (San Jose, CA); Heath, James R. (South Pasadena, CA); Phelps, Michael E. (Los Angeles, CA); Quake, Stephen R. (Stanford, CA); Tseng, Hsian-rong (Los Angeles, CA); Wyatt, Paul (Tipperary, IE); Daridon, Antoine (Mont-Sur-Rolle, CH)

2012-06-26

6

Studies of Elementary Reactions of Chemical Importance in the Atmospheres of Planets  

NASA Technical Reports Server (NTRS)

The methyl self-reaction was studied at T = 298 K and 202 K and at three different pressures, P = 0.5, 1.0, and 2.1 Torr. The experimental measurements were performed in our discharge flow-mass spectrometer (DF-MS) apparatus. The methyl radicals were generated by the reaction of F with methane. Passing a mixture of molecular fluorine, F2, in helium through a microwave cavity generated the atomic fluorine reagent. The atomic F enters the flow tube through a rear port on the flow tube. The methane reagent enters the flow tube through a movable injector located coaxial in the flow tube. The decay of methyl radical signal was monitored at a mass/charge ratio (m/z) of 15 as a function of the injector distance. To minimize secondary chemistry from the reaction CH3 + F to CH2 + HF the initial [CH4](sub 0)/[F](sub 0) was above 37.0 and typically 100. This ensures a 1:1 relationship between initial [F] and [CH3]. A titration of F with excess Cl2 yields the initial [F](sub 0). Our experimental methodology to accurately measure the mass spectrometer scaling factor, i.e., the relationship between initial signal and [CH3](sub 0) has been improved. Now we measure the CH3 signal decay under exponential decay conditions at low initial [F](sub 0), 3x10(exp 11) molecule/cc, in the presence of Cl2. This minimizes the second-order decay contributed by the CH3 self-reaction and a simple extrapolation of the 1n(signal) vs time plot to t = 0 gives the initial signal. This provides the desired relationship between initial signal at 15 amu and [CH3](sub 0). The resulting calibration is then applied to the observed decay of the CH3 signal at high concentrations of CH3 assuming linearity of this scaling factor.

Nesbitt, Fred L.

2001-01-01

7

Chemical Reactions (Netorials)  

NSDL National Science Digital Library

Chemical Reactions: this is a resource in the collection "Netorials". The Netorials cover selected topics in first-year chemistry including: Chemical Reactions, Stoichiometry, Thermodynamics, Intermolecular Forces, Acids & Bases, Biomolecules, and Electrochemistry.

8

Chemical Reactions at Surfaces  

SciTech Connect

Chemical reactions at surfaces underlie some of the most important processes of today, including catalysis, energy conversion, microelectronics, human health and the environment. Understanding surface chemical reactions at a fundamental level is at the core of the field of surface science. The Gordon Research Conference on Chemical Reactions at Surfaces is one of the premiere meetings in the field. The program this year will cover a broad range of topics, including heterogeneous catalysis and surface chemistry, surfaces in environmental chemistry and energy conversion, reactions at the liquid-solid and liquid-gas interface, electronic materials growth and surface modification, biological interfaces, and electrons and photons at surfaces. An exciting program is planned, with contributions from outstanding speakers and discussion leaders from the international scientific community. The conference provides a dynamic environment with ample time for discussion and interaction. Attendees are encouraged to present posters; the poster sessions are historically well attended and stimulate additional discussions. The conference provides an excellent opportunity for junior researchers (e.g. graduate students or postdocs) to present their work and interact with established leaders in the field.

Michael Henderson and Nancy Ryan Gray

2010-04-14

9

Chemical Reactions and Stoichiometry  

NSDL National Science Digital Library

In this activity, students explore reactions in which chemical bonds are formed and broken. Students experiment with changing the temperature and the concentration of the atoms in order to see how these affect reaction rates. They also learn how to communicate what happens during a chemical reaction by writing the ratios of reactants and products, known as stoichiometry.

The Concord Consortium

2011-12-11

10

Chemical burn or reaction  

MedlinePLUS

Chemicals that touch skin can lead to a reaction on the skin, throughout the body, or both. ... leave the person alone and watch carefully for reactions affecting the entire body. Note: If a chemical gets into the eyes, the eyes should be ...

11

Rates of Chemical Reactions  

NSDL National Science Digital Library

Science Objects are two hour on-line interactive inquiry-based content modules that help teachers better understand the science content they teach. This Science Object is the third of four Science Objects in the Chemical Reactions SciPack. It demonstrates

National Science Teachers Association (NSTA)

1900-01-01

12

Categorizing Chemical Reactions  

NSDL National Science Digital Library

Science Objects are two hour on-line interactive inquiry-based content modules that help teachers better understand the science content they teach. This Science Object is the second of four Science Objects in the Chemical Reactions SciPack. It provides an

National Science Teachers Association (NSTA)

2009-07-10

13

Chemical Reactions in DSMC  

SciTech Connect

DSMC simulations of chemically reacting gas flows have generally employed procedures that convert the macroscopic chemical rate equations to reaction cross-sections at the microscopic level. They therefore depend on the availability of experimental data that has been fitted to equations of the Arrhenius form. This paper presents a physical model for dissociation and recombination reactions and a phenomenological model for exchange and chain reactions. These are based on the vibrational states of the colliding molecules and do not require any experimentally-based data. The simplicity of the models allows the corresponding rate equations to be written down and, while these are not required for the implementation of the models, they facilitate their validation. The model is applied to a typical hypersonic atmospheric entry problem and the results are compared with the corresponding results from the traditional method. It is also used to investigate both spontaneous and forced ignition as well as the structure of a deflagration wave in an oxygen-hydrogen mixture.

Bird, G. A. [GAB Consulting Pty Ltd, 144/110 Sussex Street, Sydney NSW 2000 (Australia)

2011-05-20

14

Critical Evaluation of Chemical Reaction Rates and Collision Cross Sections of Importance in the Earth's Upper Atmosphere and the Atmospheres of Other Planets, Moons, and Comets  

NASA Astrophysics Data System (ADS)

We recommend establishment of a long-term program of critical evaluation by domain experts of the rates and cross sections of atomic and molecular processes that are needed for understanding and modeling the atmospheres in the solar system. We envision products resembling those from the ongoing JPL/NASA Panel for Data Evaluation and the efforts of the international combustion modeling community funded by US DOE and its European counterpart. Both of these endeavors already provide some important inputs for modeling the atmospheres of the Earth, planets, moons, and comets. However, their applications restrict the choice of which processes to evaluate and the temperature and pressure ranges to cover, thus leaving large gaps that need to be filled. Interestingly, an older evaluation program once filled some of these gaps. Funded by the US DoD in the 1960s-1980s, the DNA Reaction Rate Handbook provided a thorough treatment of numerous types of collisions and reactions that are important in the Earth's lower ionosphere, and the program even provided funding for new laboratory measurements. Other examples could be given, with the on-line resources at NIST being among the best, but most provide a narrower scope or less critical evaluation. What is needed is not a just a list of processes and numbers (i.e., a "database"), but rather serious comparison of the available information and specific statements from independent expert laboratory/theory data providers about what should be believed, what uncertainty to assign, and what is most in need of redetermination. The major topic areas would include the following: 1. Chemical reactions of neutral atoms and molecules in their ground electronic states 2. Ion-molecule reactions 3. Chemistry, relaxation, and radiation of electronically excited atoms and molecules 4. Vibrational and rotational relaxation and radiation 5. Photoabsorption, photodissociation, and photoionization 6. Electron-impact excitation, dissociation, ionization, and recombination 7. Energetic heavy particle excitation and charge exchange

Huestis, D. L.

2005-05-01

15

Chemical Reactions: Investigating Exothermic and Endothermic Reactions  

NSDL National Science Digital Library

This activity is an inquiry-based investigation where students discover the indicators of chemical reactions (endothermic and exothermic) by collecting data and using that data to develop a testable question for further experimentation.

16

Speeding chemical reactions by focusing  

E-print Network

We present numerical results for a chemical reaction of colloidal particles which are transported by a laminar fluid and are focused by periodic obstacles in such a way that the two components are well mixed and consequently the chemical reaction is speeded up. The roles of the various system parameters (diffusion coefficients, reaction rate, obstacles sizes) are studied. We show that focusing speeds up the reaction from the diffusion limited rate (t to the power -1/2) to very close to the perfect mixing rate, (t to the power -1).

A. M. Lacasta; L. Ramirez-Piscina; J. M. Sancho; K. Lindenberg

2012-12-13

17

Geometric description of chemical reactions  

E-print Network

We use the formalism of Geometrothermodynamics to describe chemical reactions in the context of equilibrium thermodynamics. Any chemical reaction in a closed system is shown to be described by a geodesic in a $2-$dimensional manifold that can be interpreted as the equilibrium space of the reaction. We first show this in the particular cases of a reaction with only two species corresponding to either two ideal gases or two van der Waals gases. We then consider the case of a reaction with an arbitrary number of species. The initial equilibrium state of the geodesic is determined by the initial conditions of the reaction. The final equilibrium state, which follows from a thermodynamic analysis of the reaction, is shown to correspond to a coordinate singularity of the thermodynamic metric which describes the equilibrium manifold.

Hernando Quevedo; Diego Tapias

2013-01-02

18

LEGO® Chemical Reactions  

NSDL National Science Digital Library

This activity uses LEGO® bricks to represent atoms bonding into molecules and crystals. The lesson plan is for a 2.5 hour workshop (or four 45-minute classes). There is a "wet lab" chemistry experiment (mixing baking soda and calcium chloride with phenol red indicator), followed by a "LEGO lab" modeling phase that includes writing formulas using chemical notation. This lesson is also offered as a 2.5 hour field trip lesson at the MIT Edgerton Center.

Kathleen M. Vandiver, Ph.D.

2009-01-01

19

Classes of Chemical Reactions Reactions in aqueous media  

E-print Network

Classes of Chemical Reactions Reactions in aqueous media · Precipitation reactions · Acid-Base+ , and Pb2+ #12;Classes of Chemical Reactions Acid-Base Reactions Acid - a substance that is a donor of H Acid-Base Reactions STRONG ACIDS WEAK ACIDS Hydrochloric acid, HCl Hydrofluoric acid, HF Hydrobromic

Zakarian, Armen

20

Computed potential energy surfaces for chemical reactions  

NASA Technical Reports Server (NTRS)

Quantum mechanical methods have been used to compute potential energy surfaces for chemical reactions. The reactions studied were among those believed to be important to the NASP and HSR programs and included the recombination of two H atoms with several different third bodies; the reactions in the thermal Zeldovich mechanism; the reactions of H atom with O2, N2, and NO; reactions involved in the thermal De-NO(x) process; and the reaction of CH(squared Pi) with N2 (leading to 'prompt NO'). These potential energy surfaces have been used to compute reaction rate constants and rates of unimolecular decomposition. An additional application was the calculation of transport properties of gases using a semiclassical approximation (and in the case of interactions involving hydrogen inclusion of quantum mechanical effects).

Walch, Stephen P.

1994-01-01

21

Experimental Demonstrations in Teaching Chemical Reactions.  

ERIC Educational Resources Information Center

Presents demonstrations of chemical reactions by employing different features of various compounds that can be altered after a chemical change occurs. Experimental activities include para- and dia-magnetism in chemical reactions, aluminum reaction with base, reaction of acid with carbonates, use of electrochemical cells for demonstrating chemical…

Hugerat, Muhamad; Basheer, Sobhi

2001-01-01

22

Homeostasis in Chemical Reaction Pathways  

E-print Network

We consider stochastic models of chemical reaction networks with time dependent input rates and several types of molecules. We prove that, in despite of strong time dependence of input rates, there is a kind of homeostasis phenomenon: far away from input nodes the mean numbers of molecules of each type become approximately constant (do not depend on time).

Malyshev, V A; Zamyatin, A A

2011-01-01

23

Homeostasis in Chemical Reaction Pathways  

E-print Network

We consider stochastic models of chemical reaction networks with time dependent input rates and several types of molecules. We prove that, in despite of strong time dependence of input rates, there is a kind of homeostasis phenomenon: far away from input nodes the mean numbers of molecules of each type become approximately constant (do not depend on time).

V. A. Malyshev; A. D. Manita; A. A. Zamyatin

2011-12-25

24

Visualization of chemical reaction dynamics: Toward understanding complex polyatomic reactions  

PubMed Central

Polyatomic molecules have several electronic states that have similar energies. Consequently, their chemical dynamics often involve nonadiabatic transitions between multiple potential energy surfaces. Elucidating the complex reactions of polyatomic molecules is one of the most important tasks of theoretical and experimental studies of chemical dynamics. This paper describes our recent experimental studies of the multidimensional multisurface dynamics of polyatomic molecules based on two-dimensional ion/electron imaging. It also discusses ultrafast photoelectron spectroscopy of liquids for elucidating nonadiabatic electronic dynamics in aqueous solutions. PMID:23318678

SUZUKI, Toshinori

2013-01-01

25

Mineralogical and chemical assessment of concrete damaged by the oxidation of sulfide-bearing aggregates: Importance of thaumasite formation on reaction mechanisms  

SciTech Connect

Damages in concrete containing sulfide-bearing aggregates were recently observed in the Trois-Rivieres area (Quebec, Canada), characterized by rapid deterioration within 3 to 5 years after construction. A petrographic examination of concrete core samples was carried out using a combination of tools including: stereomicroscopic evaluation, polarized light microscopy, scanning electron microscopy, X-ray diffraction and electron microprobe analysis. The aggregate used to produce concrete was an intrusive igneous rock with different metamorphism degrees and various proportions of sulfide minerals. In the rock, sulfide minerals were often surrounded by a thin layer of carbonate minerals (siderite). Secondary reaction products observed in the damaged concrete include 'rust' mineral forms (e.g. ferric oxyhydroxides such as goethite, limonite (FeO (OH) nH{sub 2}O) and ferrihydrite), gypsum, ettringite and thaumasite. In the presence of water and oxygen, pyrrhotite oxidizes to form iron oxyhydroxides and sulphuric acid. The acid then reacts with the phases of the cement paste/aggregate and provokes the formation of sulfate minerals. Understanding both mechanisms, oxidation and internal sulfate attack, is important to be able to duplicate the damaging reaction in laboratory conditions, thus allowing the development of a performance test for evaluating the potential for deleterious expansion in concrete associated with sulfide-bearing aggregates.

Rodrigues, A. [Centre de Recherche sur les Infrastructures en Beton (CRIB), Universite Laval, 1065 ave de la Medecine, Quebec, QC, Canada G1V 0A6 (Canada)] [Centre de Recherche sur les Infrastructures en Beton (CRIB), Universite Laval, 1065 ave de la Medecine, Quebec, QC, Canada G1V 0A6 (Canada); Duchesne, J., E-mail: josee.duchesne@ggl.ulaval.ca [Centre de Recherche sur les Infrastructures en Beton (CRIB), Universite Laval, 1065 ave de la Medecine, Quebec, QC, Canada G1V 0A6 (Canada); Fournier, B. [Centre de Recherche sur les Infrastructures en Beton (CRIB), Universite Laval, 1065 ave de la Medecine, Quebec, QC, Canada G1V 0A6 (Canada)] [Centre de Recherche sur les Infrastructures en Beton (CRIB), Universite Laval, 1065 ave de la Medecine, Quebec, QC, Canada G1V 0A6 (Canada); Durand, B. [Institut de recherche d'Hydro-Quebec (IREQ), 1740 boul. Lionel-Boulet, Varennes, QC, Canada J3X 1S1 (Canada)] [Institut de recherche d'Hydro-Quebec (IREQ), 1740 boul. Lionel-Boulet, Varennes, QC, Canada J3X 1S1 (Canada); Rivard, P. [Universite de Sherbrooke, Sherbrooke, QC, Canada J1K 2R1 (Canada)] [Universite de Sherbrooke, Sherbrooke, QC, Canada J1K 2R1 (Canada); Shehata, M. [Ryerson University, 350 Victoria Street, Toronto, ON, Canada M5B 2K3 (Canada)] [Ryerson University, 350 Victoria Street, Toronto, ON, Canada M5B 2K3 (Canada)

2012-10-15

26

Theoretical studies of chemical reaction dynamics  

SciTech Connect

This collaborative program with the Theoretical Chemistry Group at Argonne involves theoretical studies of gas phase chemical reactions and related energy transfer and photodissociation processes. Many of the reactions studied are of direct relevance to combustion; others are selected they provide important examples of special dynamical processes, or are of relevance to experimental measurements. Both classical trajectory and quantum reactive scattering methods are used for these studies, and the types of information determined range from thermal rate constants to state to state differential cross sections.

Schatz, G.C. [Argonne National Laboratory, IL (United States)

1993-12-01

27

Minimum Energy Pathways for Chemical Reactions  

NASA Technical Reports Server (NTRS)

Computed potential energy surfaces are often required for computation of such parameters as rate constants as a function of temperature, product branching ratios, and other detailed properties. We have found that computation of the stationary points/reaction pathways using CASSCF/derivative methods, followed by use of the internally contracted CI method to obtain accurate energetics, gives useful results for a number of chemically important systems. The talk will focus on a number of applications to reactions leading to NOx and soot formation in hydrocarbon combustion.

Walch, S. P.; Langhoff, S. R. (Technical Monitor)

1995-01-01

28

Chemical reactions at aqueous interfaces  

NASA Astrophysics Data System (ADS)

Interfaces or phase boundaries are a unique chemical environment relative to individual gas, liquid, or solid phases. Interfacial reaction mechanisms and kinetics are often at variance with homogeneous chemistry due to mass transfer, molecular orientation, and catalytic effects. Aqueous interfaces are a common subject of environmental science and engineering research, and three environmentally relevant aqueous interfaces are investigated in this thesis: 1) fluorochemical sonochemistry (bubble-water), 2) aqueous aerosol ozonation (gas-water droplet), and 3) electrolytic hydrogen production and simultaneous organic oxidation (water-metal/semiconductor). Direct interfacial analysis under environmentally relevant conditions is difficult, since most surface-specific techniques require relatively `extreme' conditions. Thus, the experimental investigations here focus on the development of chemical reactors and analytical techniques for the completion of time/concentration-dependent measurements of reactants and their products. Kinetic modeling, estimations, and/or correlations were used to extract information on interfacially relevant processes. We found that interfacial chemistry was determined to be the rate-limiting step to a subsequent series of relatively fast homogeneous reactions, for example: 1) Pyrolytic cleavage of the ionic headgroup of perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA) adsorbed to cavitating bubble-water interfaces during sonolysis was the rate-determining step in transformation to their inorganic constituents carbon monoxide, carbon dioxide, and fluoride; 2) ozone oxidation of aqueous iodide to hypoiodous acid at the aerosol-gas interface is the rate-determining step in the oxidation of bromide and chloride to dihalogens; 3) Electrolytic oxidation of anodic titanol surface groups is rate-limiting for the overall oxidation of organics by the dichloride radical. We also found chemistry unique to the interface, for example: 1) Adsorption of dilute PFOS(aq) and PFOA(aq) to acoustically cavitating bubble interfaces was greater than equilibrium expectations due to high-velocity bubble radial oscillations; 2) Relative ozone oxidation kinetics of aqueous iodide, sulfite, and thiosulfate were at variance with previously reported bulk aqueous kinetics; 3) Organics that directly chelated with the anode surface were oxidized by direct electron transfer, resulting in immediate carbon dioxide production but slower overall oxidation kinetics. Chemical reactions at aqueous interfaces can be the rate-limiting step of a reaction network and often display novel mechanisms and kinetics as compared to homogeneous chemistry.

Vecitis, Chad David

2009-12-01

29

Plasmon-driven sequential chemical reactions in an aqueous environment  

PubMed Central

Plasmon-driven sequential chemical reactions were successfully realized in an aqueous environment. In an electrochemical environment, sequential chemical reactions were driven by an applied potential and laser irradiation. Furthermore, the rate of the chemical reaction was controlled via pH, which provides indirect evidence that the hot electrons generated from plasmon decay play an important role in plasmon-driven chemical reactions. In acidic conditions, the hot electrons were captured by the abundant H+ in the aqueous environment, which prevented the chemical reaction. The developed plasmon-driven chemical reactions in an aqueous environment will significantly expand the applications of plasmon chemistry and may provide a promising avenue for green chemistry using plasmon catalysis in aqueous environments under irradiation by sunlight. PMID:24958029

Zhang, Xin; Wang, Peijie; Zhang, Zhenglong; Fang, Yurui; Sun, Mengtao

2014-01-01

30

Plasmon-driven sequential chemical reactions in an aqueous environment  

NASA Astrophysics Data System (ADS)

Plasmon-driven sequential chemical reactions were successfully realized in an aqueous environment. In an electrochemical environment, sequential chemical reactions were driven by an applied potential and laser irradiation. Furthermore, the rate of the chemical reaction was controlled via pH, which provides indirect evidence that the hot electrons generated from plasmon decay play an important role in plasmon-driven chemical reactions. In acidic conditions, the hot electrons were captured by the abundant H+ in the aqueous environment, which prevented the chemical reaction. The developed plasmon-driven chemical reactions in an aqueous environment will significantly expand the applications of plasmon chemistry and may provide a promising avenue for green chemistry using plasmon catalysis in aqueous environments under irradiation by sunlight.

Zhang, Xin; Wang, Peijie; Zhang, Zhenglong; Fang, Yurui; Sun, Mengtao

2014-06-01

31

Reaction efficiency effects on binary chemical reactions  

NASA Astrophysics Data System (ADS)

We study the effect of the variation of reaction efficiency in binary reactions. We use the well-known A + B ? 0 model, which has been extensively studied in the past. We perform simulations on this model where we vary the efficiency of reaction, i.e., when two particles meet they do not instantly react, as has been assumed in previous studies, but they react with a probability ?, where ? is in the range 0 < ? < 1. Our results show that at small ? values the system is reaction limited, but as ? increases it crosses over to a diffusion limited behavior. At early times, for small ? values, the particle density falls slower than for larger ? values. This fall-off goes over a crossover point, around the value of ? = 0.50 for high initial densities. Under a variety of conditions simulated, we find that the crossover point was dependent on the initial concentration but not on the lattice size. For intermediate and long times simulations, all ? values (in the depleted reciprocal density versus time plot) converge to the same behavior. These theoretical results are useful in models of epidemic reactions and epidemic spreading, where a contagion from one neighbor to the next is not always successful but proceeds with a certain probability, an analogous effect with the reaction probability examined in the current work.

Lazaridis, Filippos; Savara, Aditya; Argyrakis, Panos

2014-09-01

32

Detailed chemical kinetic mechanisms of ethyl methyl, methyl tert-butyl and ethyl tert-butyl ethers: The importance of uni-molecular elimination reactions  

Microsoft Academic Search

A reaction mechanism of ethyl methyl ether (EME), methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE) for pyrolysis and oxidation have been constructed using the same method applied to di-ethyl ether (DEE) in our recent work [1]. The mechanism, comprising of 1051 reactions involving 215 species, was tested against the experimental data obtained using shock tubes with good agreement.

K. Yasunaga; J. M. Simmie; H. J. Curran; T. Koike; O. Takahashi; Y. Kuraguchi; Y. Hidaka

2011-01-01

33

Dynamic Reaction Figures: An Integrative Vehicle for Understanding Chemical Reactions  

ERIC Educational Resources Information Center

A highly flexible learning tool, referred to as a dynamic reaction figure, is described. Application of these figures can (i) yield the correct chemical equation by simply following a set of menu driven directions; (ii) present the underlying "mechanism" in chemical reactions; and (iii) help to solve quantitative problems in a number of different…

Schultz, Emeric

2008-01-01

34

Nuclear physics reactions of astrophysical importance  

NASA Astrophysics Data System (ADS)

Understanding the origin of elements in the universe is one of the main goals of nuclear science and astrophysics today. Achieving this goal involves determining how the elements and their isotopes formed and being able to predict their abundances. At the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL), an experimental program has been established to use transfer reactions (such as (p,d) or (d,p)) to study the properties of many nuclei important to understanding the origins of various elements. Three measurements were done to aid in the determination of the origins of different light isotopes. Big Bang Nucleosynthesis calculations, constrained by the Wilkinson Microwave Anisotropy Probe results, produce primordial 7Li abundances almost a factor of four larger than those extrapolated from observations. Since primordial 7Li is believed to be mostly produced by the beta decay of 7Be, one proposed solution to this discrepancy is a resonant enhancement of the 7Be(d, p)2? reaction rate through the 5/2+ 16.7-MeV state in 9B. The 2H(7Be,d) 7Be reaction was used to search for such a resonance; none was observed. An upper limit on the width of the proposed resonance was deduced. 19F is believed to have formed in Asymptotic Giant Branch stars, but current models cannot reproduce the observed abundances of this nucleus. One of the key reactions responsible for the creation of 19F is 15N(?,?). Therefore, it is important to understand reactions that might destroy 15N, such as 15N(n,?). The magnitude of the 15N( n,?) reaction rate depends directly on the neutron spectroscopic factors of low-lying 16N levels. Currently the measured spectroscopic factors differ from those expected from theory by a factor of 2. A study has been done to resolve this discrepancy using the d( 15N,p) reaction. The spectroscopic factors were all found to be close to unity which is in agreement with theoretical predictions. In novae, gamma ray emission is believed to be primarily due to electron-positron annihilation, though the source of these positrons remains a mystery. The positrons are believed to originate from the beta decay of 18F due to its long half-life (t1/2 ˜ 110 min.). To date, gamma rays from this nucleus have not been observed. Therefore, studies have been made on reactions believed to destroy 18F, such as 18F(p,?) which goes through states in 19Ne. A recent study by Adekola et al. showed that a state at 6.289-MeV in 19Ne, just below the proton threshold, could have a significant impact on this reaction rate. However, the spin of this state could not be determined. To determine the spin of this sub-threshold state, a study of the 20Ne(p,d ) reaction was made using a proton beam on a carbon foil implanted with 20Ne. Due target contaminants, a spin assignment could not be made, but a new experimental design was created and is described in this dissertation.

O'Malley, Patrick D.

35

Stochastic flux analysis of chemical reaction networks  

PubMed Central

Background Chemical reaction networks provide an abstraction scheme for a broad range of models in biology and ecology. The two common means for simulating these networks are the deterministic and the stochastic approaches. The traditional deterministic approach, based on differential equations, enjoys a rich set of analysis techniques, including a treatment of reaction fluxes. However, the discrete stochastic simulations, which provide advantages in some cases, lack a quantitative treatment of network fluxes. Results We describe a method for flux analysis of chemical reaction networks, where flux is given by the flow of species between reactions in stochastic simulations of the network. Extending discrete event simulation algorithms, our method constructs several data structures, and thereby reveals a variety of statistics about resource creation and consumption during the simulation. We use these structures to quantify the causal interdependence and relative importance of the reactions at arbitrary time intervals with respect to the network fluxes. This allows us to construct reduced networks that have the same flux-behavior, and compare these networks, also with respect to their time series. We demonstrate our approach on an extended example based on a published ODE model of the same network, that is, Rho GTP-binding proteins, and on other models from biology and ecology. Conclusions We provide a fully stochastic treatment of flux analysis. As in deterministic analysis, our method delivers the network behavior in terms of species transformations. Moreover, our stochastic analysis can be applied, not only at steady state, but at arbitrary time intervals, and used to identify the flow of specific species between specific reactions. Our cases study of Rho GTP-binding proteins reveals the role played by the cyclic reverse fluxes in tuning the behavior of this network. PMID:24314153

2013-01-01

36

2005 Chemical Reactions at Surfaces  

SciTech Connect

The Gordon Research Conference (GRC) on 2005 Chemical Reactions at Surfaces was held at Ventura Beach Marriott, Ventura California from February 13, 2005 through February 18, 2005. The Conference was well-attended with 124 participants (attendees list attached). The attendees represented the spectrum of endeavor in this field coming from academia, industry, and government laboratories, both U.S. and foreign scientists, senior researchers, young investigators, and students. In designing the formal speakers program, emphasis was placed on current unpublished research and discussion of the future target areas in this field. There was a conscious effort to stimulate lively discussion about the key issues in the field today. Time for formal presentations was limited in the interest of group discussions. In order that more scientists could communicate their most recent results, poster presentation time was scheduled. Attached is a copy of the formal schedule and speaker program and the poster program. In addition to these formal interactions, 'free time' was scheduled to allow informal discussions. Such discussions are fostering new collaborations and joint efforts in the field.

Cynthia M. Friend

2006-03-14

37

Chemical reactions in low-g  

NASA Technical Reports Server (NTRS)

The Apollo-Soyuz flight experiment, 'Chemical Foams' demonstrated that foams and air/liquid dispersions are much more stable in low-gravity than on the ground. It thus should be possible to conduct unique chemical reactions in space foams. The low-g results and subsequent ground work on the formaldehyde clock reaction indicate that the reaction is strongly influenced by (1) dissociated and undissociated solution species being adsorbed at solid/liquid and gas/liquid surfaces and (2) chemical reaction rates apparently being affected by long-range forces determined by the liquid mass and the extent and nature of all surface interfaces.

Grodzka, P. G.; Facemire, B. R.

1978-01-01

38

Microfabricated electrochemiluminescence cell for chemical reaction detection  

DOEpatents

A detector cell for a silicon-based or non-silicon-based sleeve type chemical reaction chamber that combines heaters, such as doped polysilicon for heating, and bulk silicon for convection cooling. The detector cell is an electrochemiluminescence cell constructed of layers of silicon with a cover layer of glass, with spaced electrodes located intermediate various layers forming the cell. The cell includes a cavity formed therein and fluid inlets for directing reaction fluid therein. The reaction chamber and detector cell may be utilized in any chemical reaction system for synthesis or processing of organic, inorganic, or biochemical reactions, such as the polymerase chain reaction (PCR) and/or other DNA reactions, such as the ligase chain reaction, which are examples of a synthetic, thermal-cycling-based reaction. The ECL cell may also be used in synthesis instruments, particularly those for DNA amplification and synthesis.

Northrup, M. Allen (Berkeley, CA); Hsueh, Yun-Tai (Davis, CA); Smith, Rosemary L. (Davis, CA)

2003-01-01

39

Chemical reaction networks: Colour by number.  

PubMed

Using chemical reactions and diffusion to control pattern formation requires the careful design of reaction networks and a balance of kinetics that is difficult to achieve. Now, it has been shown that DNA-based reaction networks provide a robust method for transforming patterns. PMID:24256857

Scalise, Dominic; Schulman, Rebecca

2013-12-01

40

'GREENER' CHEMICAL SYNTHESES USING ALTERNATE REACTION CONDITIONS  

EPA Science Inventory

Microwave (MW) irradiation in conjunction with water as reaction media has proven to be a greener chemical approach for expeditious N-alkylation reactions of amines and hydrazines wherein the reactions under mildly basic conditions afford tertiary amines and double N-alkylation t...

41

Chemical Kinetics: Rate of Reaction  

NSDL National Science Digital Library

This site offers an interactive tutorial that emphasizes graphical interpretation of chemical kinetics. The stoichiometric coefficients for a chemical equation are determined by comparing the slopes of concentration-time plots for the reactants and products. This tutorial is coupled to others to further guide the student to a better understanding of chemical kinetics.

David N. Blauch

42

Chemical reactions of organic compounds on clay surfaces.  

PubMed Central

Chemical reactions of organic compounds including pesticides at the interlayer and exterior surfaces of clay minerals and with soil organic matter are reviewed. Representative reactions under moderate conditions possibly occurring in natural soils are described. Attempts have been made to clarify the importance of the chemical nature of molecules, their structures and their functional groups, and the Brönsted or Lewis acidity of clay minerals. PMID:2533556

Soma, Y; Soma, M

1989-01-01

43

Microfabricated sleeve devices for chemical reactions  

DOEpatents

A silicon-based sleeve type chemical reaction chamber that combines heaters, such as doped polysilicon for heating, and bulk silicon for convection cooling. The reaction chamber combines a critical ratio of silicon and non-silicon based materials to provide the thermal properties desired. For example, the chamber may combine a critical ratio of silicon and silicon nitride to the volume of material to be heated (e.g., a liquid) in order to provide uniform heating, yet low power requirements. The reaction chamber will also allow the introduction of a secondary tube (e.g., plastic) into the reaction sleeve that contains the reaction mixture thereby alleviating any potential materials incompatibility issues. The reaction chamber may be utilized in any chemical reaction system for synthesis or processing of organic, inorganic, or biochemical reactions, such as the polymerase chain reaction (PCR) and/or other DNA reactions, such as the ligase chain reaction, which are examples of a synthetic, thermal-cycling-based reaction. The reaction chamber may also be used in synthesis instruments, particularly those for DNA amplification and synthesis.

Northrup, M. Allen (Berkeley, CA)

2003-01-01

44

40 CFR 707.20 - Chemical substances import policy.  

Code of Federal Regulations, 2011 CFR

...2011-07-01 2011-07-01 false Chemical substances import policy. 707.20...CONTINUED) TOXIC SUBSTANCES CONTROL ACT CHEMICAL IMPORTS AND EXPORTS General Import Requirements and Restrictions § 707.20 Chemical substances import policy. (a)...

2011-07-01

45

40 CFR 707.20 - Chemical substances import policy.  

Code of Federal Regulations, 2010 CFR

...2010-07-01 2010-07-01 false Chemical substances import policy. 707.20...CONTINUED) TOXIC SUBSTANCES CONTROL ACT CHEMICAL IMPORTS AND EXPORTS General Import Requirements and Restrictions § 707.20 Chemical substances import policy. (a)...

2010-07-01

46

FAST ELEMENTARY STEPS IN CHEMICAL REACTION MECHANSIMS  

Microsoft Academic Search

A review is presented of studies of fast chemical reaction steps which ; were studied using various relaxation techniques. These studies include ligand ; substitution in metal complexes, proton transfer, and enzymatic hydrolysis. ; (D.L.C.);

M. Eigen

1963-01-01

47

Kinetic studies of elementary chemical reactions  

SciTech Connect

This program concerning kinetic studies of elementary chemical reactions is presently focussed on understanding reactions of NH{sub x} species. To reach this goal, the author is pursuing experimental studies of reaction rate coefficients and product branching fractions as well as using electronic structure calculations to calculate transition state properties and reaction rate calculations to relate these properties to predicted kinetic behavior. The synergy existing between the experimental and theoretical studies allow one to gain a deeper insight into more complex elementary reactions.

Durant, J.L. Jr. [Sandia National Laboratories, Livermore, CA (United States)

1993-12-01

48

Chemical Principles Revisited: Annotating Reaction Equations.  

ERIC Educational Resources Information Center

Urges chemistry teachers to have students annotate the chemical reactions in aqueous-solutions that they see in their textbooks and witness in the laboratory. Suggests this will help students recognize the reaction type more readily. Examples are given for gas formation, precipitate formation, redox interaction, acid-base interaction, and…

Tykodi, R. J.

1987-01-01

49

Entropy Generation in a Chemical Reaction  

ERIC Educational Resources Information Center

Entropy generation in a chemical reaction is analysed without using the general formalism of non-equilibrium thermodynamics at a level adequate for advanced undergraduates. In a first approach to the problem, the phenomenological kinetic equation of an elementary first-order reaction is used to show that entropy production is always positive. A…

Miranda, E. N.

2010-01-01

50

Fluid flow and chemical reaction kinetics in metamorphic systems  

SciTech Connect

The treatment and effects of chemical reaction kinetics during metamorphism are developed along with the incorporation of fluid flow, diffusion, and thermal evolution. The interplay of fluid flow and surface reaction rates, the distinction between steady state and equilibrium, and the possible overstepping of metamorphic reactions are discussed using a simple analytic model. This model serves as an introduction to the second part of the paper, which develops a reaction model that solves the coupled temperature-fluid flow-chemical composition differential equations relevant to metamorphic processes. Consideration of stable isotopic evidence requires that such a kinetic model be considered for the chemical evolution of a metamorphic aureole. A general numerical scheme is discussed to handle the solution of the model. The results of this kinetic model allow us to reach several important conclusions regarding the factors controlling the chemical evolution of mineral assemblages during a metamorphic event. 41 refs., 19 figs., 5 tabs.

Lasaga, A.C.; Rye, D.M. (Yale Univ., New Haven, CT (United States))

1993-05-01

51

Chemical kinetic reaction mechanism for the combustion of propane  

NASA Technical Reports Server (NTRS)

A detailed chemical kinetic reaction mechanism for the combustion of propane is presented and discussed. The mechanism consists of 27 chemical species and 83 elementary chemical reactions. Ignition and combustion data as determined in shock tube studies were used to evaluate the mechanism. Numerical simulation of the shock tube experiments showed that the kinetic behavior predicted by the mechanism for stoichiometric mixtures is in good agrement with the experimental results over the entire temperature range examined (1150-2600K). Sensitivity and theoretical studies carried out using the mechanism revealed that hydrocarbon reactions which are involved in the formation of the HO2 radical and the H2O2 molecule are very important in the mechanism and that the observed nonlinear behavior of ignition delay time with decreasing temperature can be interpreted in terms of the increased importance of the HO2 and H2O2 reactions at the lower temperatures.

Jachimowski, C. J.

1984-01-01

52

Modeling of turbulent chemical reaction  

NASA Technical Reports Server (NTRS)

Viewgraphs are presented on modeling turbulent reacting flows, regimes of turbulent combustion, regimes of premixed and regimes of non-premixed turbulent combustion, chemical closure models, flamelet model, conditional moment closure (CMC), NO(x) emissions from turbulent H2 jet flames, probability density function (PDF), departures from chemical equilibrium, mixing models for PDF methods, comparison of predicted and measured H2O mass fractions in turbulent nonpremixed jet flames, experimental evidence of preferential diffusion in turbulent jet flames, and computation of turbulent reacting flows.

Chen, J.-Y.

1995-01-01

53

Quantum Chemical Approach to Estimating the Thermodynamics of Metabolic Reactions  

NASA Astrophysics Data System (ADS)

Thermodynamics plays an increasingly important role in modeling and engineering metabolism. We present the first nonempirical computational method for estimating standard Gibbs reaction energies of metabolic reactions based on quantum chemistry, which can help fill in the gaps in the existing thermodynamic data. When applied to a test set of reactions from core metabolism, the quantum chemical approach is comparable in accuracy to group contribution methods for isomerization and group transfer reactions and for reactions not including multiply charged anions. The errors in standard Gibbs reaction energy estimates are correlated with the charges of the participating molecules. The quantum chemical approach is amenable to systematic improvements and holds potential for providing thermodynamic data for all of metabolism.

Jinich, Adrian; Rappoport, Dmitrij; Dunn, Ian; Sanchez-Lengeling, Benjamin; Olivares-Amaya, Roberto; Noor, Elad; Even, Arren Bar; Aspuru-Guzik, Alán

2014-11-01

54

Chemical Reactions Among Indoor Pollutants  

Microsoft Academic Search

\\u000a Chemistry takes place all around us, regulating the intensity and nature of our exposure to pollutants in water, air and soil.\\u000a In indoor environments, chemistry can significantly alter the composition of the air we breathe. Transformations reduce our\\u000a exposure to reactants and increase our exposure to products. If this reaction takes place on or in a surface, the relative\\u000a exposure

Glenn Morrison

55

Controlling chemical reactions of a single particle  

E-print Network

The control of chemical reactions is a recurring theme in physics and chemistry. Traditionally, chemical reactions have been investigated by tuning thermodynamic parameters, such as temperature or pressure. More recently, physical methods such as laser or magnetic field control have emerged to provide completely new experimental possibilities, in particular in the realm of cold collisions. The control of reaction pathways is also a critical component to implement molecular quantum information processing. For these undertakings, single particles provide a clean and well-controlled experimental system. Here, we report on the experimental tuning of the exchange reaction rates of a single trapped ion with ultracold neutral atoms by exerting control over both their quantum states. We observe the influence of the hyperfine interaction on chemical reaction rates and branching ratios, and monitor the kinematics of the reaction products. These investigations advance chemistry with single trapped particles towards achieving quantum-limited control of chemical reactions and indicate limits for buffer gas cooling of single ion clocks.

Lothar Ratschbacher; Christoph Zipkes; Carlo Sias; Michael Köhl

2012-09-26

56

Entropy generation in a chemical reaction  

E-print Network

Entropy generation in a chemical reaction is analyzed without using the general formalism of non-equilibrium thermodynamics at a level adequate for advanced undergraduates. In a first approach to the problem, the phenomenological kinetic equation of an elementary first order reaction is used to show that entropy production is always positive. A second approach assumes that the reaction is near equilibrium to prove that the entropy generated is always greater than zero, without any reference to the kinetics of the reaction. Finally, it is shown that entropy generation is related to fluctuations in the number of particles at equilibrium, i.e. it is associated to a microscopic process.

E. N. Miranda

2012-08-10

57

Computed potential energy surfaces for chemical reactions  

NASA Technical Reports Server (NTRS)

The objective was to obtain accurate potential energy surfaces (PES's) for a number of reactions which are important in the H/N/O combustion process. The interest in this is centered around the design of the SCRAM jet engine for the National Aerospace Plane (NASP), which was envisioned as an air-breathing hydrogen-burning vehicle capable of reaching velocities as large as Mach 25. Preliminary studies indicated that the supersonic flow in the combustor region of the scram jet engine required accurate reaction rate data for reactions in the H/N/O system, some of which was not readily available from experiment. The most important class of combustion reactions from the standpoint of the NASP project are radical recombinaton reactions, since these reactions result in most of the heat release in the combustion process. Theoretical characterizations of the potential energy surfaces for these reactions are presented and discussed.

Walch, Stephen P.

1990-01-01

58

Stochastic thermodynamics of chemical reaction networks  

E-print Network

For chemical reaction networks described by a master equation, we define energy and entropy on a stochastic trajectory and develop a consistent nonequilibrium thermodynamic description along a single stochastic trajectory of reaction events. A first-law like energy balance relates internal energy, applied (chemical) work and dissipated heat for every single reaction. Entropy production along a single trajectory involves a sum over changes in the entropy of the network itself and the entropy of the medium. The latter is given by the exchanged heat identified through the first law. Total entropy production is constrained by an integral fluctuation theorem for networks arbitrarily driven by time-dependent rates and a detailed fluctuation theorem for networks in the steady state. Further exact relations like a generalized Jarzynski relation and a generalized Clausius inequality are discussed. We illustrate these results for a three-species cyclic reaction network which exhibits nonequilibrium steady states as well as transitions between different steady states.

Tim Schmiedl; Udo Seifert

2006-12-19

59

Quantum theory of bimolecular chemical reactions  

Microsoft Academic Search

In this review we discuss quantum dynamically based theoretical methods for studying bimolecular gas phase chemical reactions. The scope is largely limited to reactions occurring on a single Born-Oppenheimer potential energy surface and mainly to time-independent Hamiltonians. An introductory overview is given, which includes a general discussion on approaches aiming to solve the time-independent and the time-dependent Schrdinger equation respectively,

Gunnar Nyman; Hua-Yu Gen

2000-01-01

60

Aerosol simulation including chemical and nuclear reactions  

SciTech Connect

The numerical simulation of aerosol transport, including the effects of chemical and nuclear reactions presents a challenging dynamic accounting problem. Particles of different sizes agglomerate and settle out due to various mechanisms, such as diffusion, diffusiophoresis, thermophoresis, gravitational settling, turbulent acceleration, and centrifugal acceleration. Particles also change size, due to the condensation and evaporation of materials on the particle. Heterogeneous chemical reactions occur at the interface between a particle and the suspending medium, or a surface and the gas in the aerosol. Homogeneous chemical reactions occur within the aersol suspending medium, within a particle, and on a surface. These reactions may include a phase change. Nuclear reactions occur in all locations. These spontaneous transmutations from one element form to another occur at greatly varying rates and may result in phase or chemical changes which complicate the accounting process. This paper presents an approach for inclusion of these effects on the transport of aerosols. The accounting system is very complex and results in a large set of stiff ordinary differential equations (ODEs). The techniques for numerical solution of these ODEs require special attention to achieve their solution in an efficient and affordable manner. 4 refs.

Marwil, E.S.; Lemmon, E.C.

1985-01-01

61

Instantaneous Chemical Reactions in Benzene and Toluene  

E-print Network

KU ScholarWorks | The University of Kansas Pre-1923 Dissertations and Theses Collection Instantaneous Chemical Reac- tions in Benzene and Toluene June 7th, 1905 by Herman Camp Allen This work was digitized by the Scholarly Communications program... Chemistry Allen, H.C. 1905 "Instantaneous reactions (chemical) in benzene and toluene". I ! B f O H B M I O A L REACTIONS IN Bt«2F;»F! AND TQLUBMB, Presented to the faculty of the University of Kansas in partial fulfillment of the requirements...

Allen, Herman Camp

1905-06-07

62

Decrease of Entropy and Chemical Reactions  

E-print Network

The chemical reactions are very complex, and include oscillation, condensation, catalyst and self-organization, etc. In these case changes of entropy may increase or decrease. The second law of thermodynamics is based on an isolated system and statistical independence. If fluctuations magnified due to internal interactions exist in the system, entropy will decrease possibly. In chemical reactions there are various internal interactions, so that some ordering processes with decrease of entropy are possible on an isolated system. For example, a simplifying Fokker-Planck equation is solved, and the hysteresis as limit cycle is discussed.

Yi-Fang Chang

2008-07-01

63

Acceleration of chemical reaction by chaotic mixing  

E-print Network

Theory of fast binary chemical reaction, ${\\cal A}+{\\cal B}\\to{\\cal C}$, in a statistically stationary chaotic flow at large Schmidt number ${Sc}$ and large Damk\\"ohler number ${Da}$ is developed. For stoichiometric condition we identify subsequent stages of the chemical reaction. The first stage corresponds to the exponential decay, $\\propto\\exp(-\\lambda t)$ (where $\\lambda$ is the Lyapunov exponent of the flow), of the chemicals in the bulk part of the flow. The second and the third stages are related to the chemicals remaining in the boundary region. During the second stage the amounts of ${\\cal A}$ and ${\\cal B}$ decay $\\propto 1/\\sqrt{t}$, whereas the decay law during the third stage is exponential, $\\propto\\exp(-\\gamma t)$, where $\\gamma\\sim\\lambda/\\sqrt{Sc}$.

M. Chertkov; V. Lebedev

2003-01-27

64

Electronic energy density in chemical reaction systems  

NASA Astrophysics Data System (ADS)

The energy of chemical reaction is visualized in real space using the electronic energy density nE(r?) associated with the electron density n(r?). The electronic energy density nE(r?) is decomposed into the kinetic energy density nT(r?), the external potential energy density nV(r?), and the interelectron potential energy density nW(r?). Using the electronic energy density nE(r?) we can pick up any point in a chemical reaction system and find how the electronic energy E is assigned to the selected point. We can then integrate the electronic energy density nE(r?) in any region R surrounding the point and find out the regional electronic energy ER to the global E. The kinetic energy density nT(r?) is used to identify the intrinsic shape of the reactants, the electronic transition state, and the reaction products along the course of the chemical reaction coordinate. The intrinsic shape is identified with the electronic interface S that discriminates the region RD of the electronic drop from the region RA of the electronic atmosphere in the density distribution of the electron gas. If the R spans the whole space, then the integral gives the total E. The regional electronic energy ER in thermodynamic ensemble is realized in electrochemistry as the intrinsic Volta electric potential ?R and the intrinsic Herring-Nichols work function ?R. We have picked up first a hydrogen-like atom for which we have analytical exact expressions of the relativistic kinetic energy density nTM(r?) and its nonrelativistic version nT(r?). These expressions are valid for any excited bound states as well as the ground state. Second, we have selected the following five reaction systems and show the figures of the nT(r?) as well as the other energy densities along the intrinsic reaction coordinates: a protonation reaction to He, addition reactions of HF to C2H4 and C2H2, hydrogen abstraction reactions of NH3+ from HF and NH3. Valence electrons possess their unique delocalized drop region remote from those heavily localized drop regions adhered to core electrons. The kinetic energy density nT(r?) and the tension density ??S(r?) can vividly demonstrate the formation of the chemical bond. Various basic chemical concepts in these chemical reaction systems have been clearly visualized in real three-dimensional space.

Tachibana, Akitomo

2001-08-01

65

40 CFR 799.19 - Chemical imports and exports.  

Code of Federal Regulations, 2011 CFR

...2011-07-01 2011-07-01 false Chemical imports and exports. 799.19 Section...CONTINUED) IDENTIFICATION OF SPECIFIC CHEMICAL SUBSTANCE AND MIXTURE TESTING REQUIREMENTS General Provisions § 799.19 Chemical imports and exports. Persons...

2011-07-01

66

40 CFR 799.19 - Chemical imports and exports.  

Code of Federal Regulations, 2010 CFR

...2010-07-01 2010-07-01 true Chemical imports and exports. 799.19 Section...CONTINUED) IDENTIFICATION OF SPECIFIC CHEMICAL SUBSTANCE AND MIXTURE TESTING REQUIREMENTS General Provisions § 799.19 Chemical imports and exports. Persons...

2010-07-01

67

Chemical Demonstrations with Consumer Chemicals: The Black and White Reaction  

NASA Astrophysics Data System (ADS)

A color-change reaction is described in which two colorless solutions are combined to afford a black mixture. Two more colorless solutions are combined to afford a white mixture. The black and white mixtures are then combined to afford a clear, colorless solution. The reaction uses chemicals that are readily available on the retail market: vitamin C, tincture of iodine, vinegar, ammonia, bleach, Epsom salt, and laundry starch.

Wright, Stephen W.

2002-01-01

68

Basic Chemical Principles 1: Reaction Kinetics  

E-print Network

of reactants ! mobility (masses) and density { Di#11;usion-controlled reactions: prefactor A is the most that Oxygen atom and molecule O 2 are ground state triplets. #15; Important fact: Quantum mechanics tell us: Spin forbidden #15; Consequence: Reactions with ground state Oxygen are slow #15; Molecules can

Schofield, Jeremy

69

Classification of Chemical Reactions: Stages of Expertise  

ERIC Educational Resources Information Center

In this study we explore the strategies that undergraduate and graduate chemistry students use when engaged in classification tasks involving symbolic and microscopic (particulate) representations of different chemical reactions. We were specifically interested in characterizing the basic features to which students pay attention when classifying…

Stains, Marilyne; Talanquer, Vicente

2008-01-01

70

Chemical reactions in reverse micelle systems  

DOEpatents

This invention is directed to conducting chemical reactions in reverse micelle or microemulsion systems comprising a substantially discontinuous phase including a polar fluid, typically an aqueous fluid, and a microemulsion promoter, typically a surfactant, for facilitating the formation of reverse micelles in the system. The system further includes a substantially continuous phase including a non-polar or low-polarity fluid material which is a gas under standard temperature and pressure and has a critical density, and which is generally a water-insoluble fluid in a near critical or supercritical state. Thus, the microemulsion system is maintained at a pressure and temperature such that the density of the non-polar or low-polarity fluid exceeds the critical density thereof. The method of carrying out chemical reactions generally comprises forming a first reverse micelle system including an aqueous fluid including reverse micelles in a water-insoluble fluid in the supercritical state. Then, a first reactant is introduced into the first reverse micelle system, and a chemical reaction is carried out with the first reactant to form a reaction product. In general, the first reactant can be incorporated into, and the product formed in, the reverse micelles. A second reactant can also be incorporated in the first reverse micelle system which is capable of reacting with the first reactant to form a product.

Matson, Dean W. (Kennewick, WA); Fulton, John L. (Richland, WA); Smith, Richard D. (Richland, WA); Consani, Keith A. (Richland, WA)

1993-08-24

71

BSHS Calculus 2: 10. Modeling Chemical ReactionsBSHS Calculus 2: 10. Modeling Chemical Reactions  

NSDL National Science Digital Library

The law of mass action has its origin in modeling chemical reactions. Michaelis and Menten (1913) simplified assumptions. A more realistic model was developed by Briggs and Haldane (1925). Other models including gene regulatory networks will be examined.

Claudia Neuhauser (University of Minnesota Rochester; Health Sciences)

2008-11-18

72

Semiclassical approaches to controlling chemical reaction dynamics  

E-print Network

We propose to use semiclassical methods to treat laser control problems of chemical reaction dynamics. Our basic strategy is as follows: Laser-driven chemical reactions are considered to consist of two processes. One is the wavepacket propagation on an adiabatic potential energy surface (PES), and the other is the electronic transition between PES's. Because the latter process is mathematically equivalent to nonadiabatic transitions between Floquet (dressed) states, we can control such a process using the semiclassical Zhu-Nakamura theory for nonadiabatic transitions. For the former process, we incorporate semiclassical propagation methods such as the Herman-Kluk propagator into optimization procedures like optimal control theory. We show some numerical examples for our strategies. We also develop a semiclassical direct algorithm to treat the adiabatic propagation and nonadiabatic transitions as a whole.

Hiroshi Fujisaki; Yoshiaki Teranishi; Alexey Kondorskiy; Hiroki Nakamura

2003-02-04

73

Researches on Preliminary Chemical Reactions in Spark-Ignition Engines  

NASA Technical Reports Server (NTRS)

Chemical reactions can demonstrably occur in a fuel-air mixture compressed in the working cylinder of an Otto-cycle (spark ignition) internal-combustion engine even before the charge is ignited by the flame proceeding from the sparking plug. These are the so-called "prelinminary reactions" ("pre-flame" combustion or oxidation), and an exact knowledge of their characteristic development is of great importance for a correct appreciation of the phenomena of engine-knock (detonation), and consequently for its avoidance. Such reactions can be studied either in a working engine cylinder or in a combustion bomb. The first method necessitates a complicated experimental technique, while the second has the disadvantage of enabling only a single reaction to be studied at one time. Consequently, a new series of experiments was inaugurated, conducted in a motored (externally-driven) experimental engine of mixture-compression type, without ignition, the resulting preliminary reactions being detectable and measurable thermometrically.

Muehlner, E.

1943-01-01

74

The role of chemical reactions in the Chernobyl accident  

SciTech Connect

It is shown that chemical reactions played an essential role in the Chernobyl accident at all of its stages. It is important that the reactor before the explosion was at maximal xenon poisoning, and its reactivity, apparently, was not destroyed by the explosion. The reactivity release due to decay of Xe-235 on the second day after the explosion led to a reactor power of 80-110 MW. Owing to this power, the chemical reactions of reduction of uranium, plutonium, and other metals at a temperature of about 2000 Degree-Sign C occurred in the core. The yield of fission products thus sharply increased. Uranium and other metals flew down in the bottom water communications and rooms. After reduction of the uranium and its separation from the graphite, the chain reaction stopped, the temperature of the core decreased, and the activity yield stopped.

Grishanin, E. I., E-mail: egrishanin@orexovo.net [Russian Research Center Kurchatov Institute (Russian Federation)

2010-12-15

75

Multiscale stochastic simulations of chemical reactions with regulated scale separation  

SciTech Connect

We present a coupling of multiscale frameworks with accelerated stochastic simulation algorithms for systems of chemical reactions with disparate propensities. The algorithms regulate the propensities of the fast and slow reactions of the system, using alternating micro and macro sub-steps simulated with accelerated algorithms such as ? and R-leaping. The proposed algorithms are shown to provide significant speedups in simulations of stiff systems of chemical reactions with a trade-off in accuracy as controlled by a regulating parameter. More importantly, the error of the methods exhibits a cutoff phenomenon that allows for optimal parameter choices. Numerical experiments demonstrate that hybrid algorithms involving accelerated stochastic simulations can be, in certain cases, more accurate while faster, than their corresponding stochastic simulation algorithm counterparts.

Koumoutsakos, Petros, E-mail: petros@ethz.ch [Chair of Computational Science, Clausiusstrasse 33, ETH Zurich, CH-8092 (Switzerland)] [Chair of Computational Science, Clausiusstrasse 33, ETH Zurich, CH-8092 (Switzerland); Feigelman, Justin [Chair of Computational Science, Clausiusstrasse 33, ETH Zurich, CH-8092 (Switzerland)] [Chair of Computational Science, Clausiusstrasse 33, ETH Zurich, CH-8092 (Switzerland)

2013-07-01

76

Density functional study of chemical reaction equilibrium for dimerization reactions in slit and cylindrical nanopores  

E-print Network

Density functional study of chemical reaction equilibrium for dimerization reactions in slit a theoretical study of the effects of confinement on chemical reaction equilibrium in slit and cylindrical equilibrium, for which much less is known. The behavior of chemical reactions in confinement spans a wide

Lisal, Martin

77

Chemical reaction equilibrium in nanoporous materials: NO dimerization reaction in carbon slit nanopores  

E-print Network

Chemical reaction equilibrium in nanoporous materials: NO dimerization reaction in carbon slit of confinement on chemical reaction equilibrium in nanoporous materials. We use the reaction ensemble Monte Carlo nanopores Martin Lísala E. Hála Laboratory of Thermodynamics, Institute of Chemical Process Fundamentals

Lisal, Martin

78

Molecular Dynamics Simulations of Chemical Reactions for Use in Education  

ERIC Educational Resources Information Center

One of the simulation engines of an open-source program called the Molecular Workbench, which can simulate thermodynamics of chemical reactions, is described. This type of real-time, interactive simulation and visualization of chemical reactions at the atomic scale could help students understand the connections between chemical reaction equations…

Qian Xie; Tinker, Robert

2006-01-01

79

Chemical Reaction due to Stronger Ramachandran Interaction  

E-print Network

The origin of a chemical reaction between two reactant atoms is associated to the activation energy, with the assumption that, high-energy collisions between these atoms, are the ones that overcome the activation energy. Here, we (i) show that a stronger attractive van der Waals (vdW) and electron-ion Coulomb interactions between two polarized atoms are responsible to initiate a chemical reaction, either before or after the collision. We derive this stronger vdW attraction formula exactly using the quasi one-dimensional Drude model within the ionization energy theory and the energy-level spacing renormalization group method. Along the way, we (ii) expose the precise physical mechanism responsible for the existence of a stronger vdW interaction for both long and short distances, and also show how to technically avoid the electron-electron Coulomb repulsion between polarized electrons from these two reactant atoms. Finally, we properly and correctly associate the existence of this stronger attraction to Ramachandran's 'normal limits' (distance shorter than what is allowed by the standard vdW bond) between chemically nonbonded atoms.

Andrew Das Arulsamy

2013-12-26

80

Chemical Looping Combustion Reactions and Systems  

SciTech Connect

Chemical Looping Combustion (CLC) is one promising fuel-combustion technology, which can facilitate economic CO{sub 2} capture in coal-fired power plants. It employs the oxidation/reduction characteristics of a metal, or oxygen carrier, and its oxide, the oxidizing gas (typically air) and the fuel source may be kept separate. This topical report discusses the results of four complementary efforts: (5.1) the development of process and economic models to optimize important design considerations, such as oxygen carrier circulation rate, temperature, residence time; (5.2) the development of high-performance simulation capabilities for fluidized beds and the collection, parameter identification, and preliminary verification/uncertainty quantification; (5.3) the exploration of operating characteristics in the laboratoryscale bubbling bed reactor, with a focus on the oxygen carrier performance, including reactivity, oxygen carrying capacity, attrition resistance, resistance to deactivation, cost and availability; and (5.4) the identification of kinetic data for copper-based oxygen carriers as well as the development and analysis of supported copper oxygen carrier material. Subtask 5.1 focused on the development of kinetic expressions for the Chemical Looping with Oxygen Uncoupling (CLOU) process and validating them with reported literature data. The kinetic expressions were incorporated into a process model for determination of reactor size and oxygen carrier circulation for the CLOU process using ASPEN PLUS. An ASPEN PLUS process model was also developed using literature data for the CLC process employing an iron-based oxygen carrier, and the results of the process model have been utilized to perform a relative economic comparison. In Subtask 5.2, the investigators studied the trade-off between modeling approaches and available simulations tools. They quantified uncertainty in the high-performance computing (HPC) simulation tools for CLC bed applications. Furthermore, they performed a sensitivity analysis for velocity, height and polydispersity and compared results against literature data for experimental studies of CLC beds with no reaction. Finally, they present an optimization space using simple non-reactive configurations. In Subtask 5.3, through a series of experimental studies, behavior of a variety of oxygen carriers with different loadings and manufacturing techniques was evaluated under both oxidizing and reducing conditions. The influences of temperature, degree of carrier conversion and thermodynamic driving force resulting from the difference between equilibrium and system O{sub 2} partial pressures were evaluated through several experimental campaigns, and generalized models accounting for these influences were developed to describe oxidation and oxygen release. Conversion of three solid fuels with widely ranging reactivities was studied in a small fluidized bed system, and all but the least reactive fuel (petcoke) were rapidly converted by oxygen liberated from the CLOU carrier. Attrition propensity of a variety of carriers was also studied, and the carriers produced by freeze granulation or impregnation of preformed substrates displayed the lowest rates of attrition. Subtask 5.4 focused on gathering kinetic data for a copper-based oxygen carrier to assist with modeling of a functioning chemical looping reactor. The kinetics team was also responsible for the development and analysis of supported copper oxygen carrier material.

Sarofim, Adel; Lighty, JoAnn; Smith, Philip; Whitty, Kevin; Eyring, Edward; Sahir, Asad; Alvarez, Milo; Hradisky, Michael; Clayton, Chris; Konya, Gabor; Baracki, Richard; Kelly, Kerry

2014-03-01

81

Quantum dynamics of fast chemical reactions  

SciTech Connect

The aims of this research are to explore, develop, and apply theoretical methods for the evaluation of the dynamics of gas phase collision processes, primarily chemical reactions. The primary theoretical tools developed for this work have been quantum scattering theory, both in time dependent and time independent forms. Over the past several years, the authors have developed and applied methods for the direct quantum evaluation of thermal rate constants, applying these to the evaluation of the hydrogen isotopic exchange reactions, applied wave packet propagation techniques to the dissociation of Rydberg H{sub 3}, incorporated optical potentials into the evaluation of thermal rate constants, evaluated the use of optical potentials for state-to-state reaction probability evaluations, and, most recently, have developed quantum approaches for electronically non-adiabatic reactions which may be applied to simplify calculations of reactive, but electronically adiabatic systems. Evaluation of the thermal rate constants and the dissociation of H{sub 3} were reported last year, and have now been published.

Light, J.C. [Univ. of Chicago, IL (United States)

1993-12-01

82

Sonoluminescence air bubbles as chemical reaction chambers  

E-print Network

Sound driven gas bubbles can emit light pulses, a phenomenon called sonoluminescence. Air is found to be one of the most friendly gases towards this phenomenon, but only if it contains 1\\% argon. We suggest a chemical mechanism to account for the strong dependence on the gas mixture, based on the dissociation of nitrogen at high temperatures and reactions which form \\rm{NO}_3^- and \\rm{NH}_4^+, among other ions; the reaction products should be investigated experimentally. Inert gases are crucial for stable sonoluminescence because they do not react with the fluid. Our phase diagram in the concentration vs forcing pressure space is applicable to any gas mixture and in good agreement with latest measurements of the UCLA group.

Lohse, D; Dupont, T F; Hilgenfeldt, S; Johnston, B; Lohse, Detlef; Brenner, Michael P; Dupont, Todd F; Hilgenfeldt, Sascha; Johnston, Blaine

1996-01-01

83

On the geometrical thermodynamics of chemical reactions  

E-print Network

The formal structure of geometrical thermodynamics is reviewed with particular emphasis on the geometry of equilibria submanifolds. On these submanifolds thermodynamic metrics are defined as the Hessian of thermodynamic potentials. Links between geometry and thermodynamics are explored for single and multiple component, closed and open systems. For multi-component closed and open systems the Gibbs free energy is employed as the thermodynamic potential to investigate the connection between geometry and thermodynamics. The Gibbs free energy is chosen for the analysis of multicomponent systems and, in particular, chemical reactions.

Manuel Santoro; Albert S. Benight

2005-07-08

84

Stochastic Chemical Reactions in Micro-domains  

E-print Network

Traditional chemical kinetics may be inappropriate to describe chemical reactions in micro-domains involving only a small number of substrate and reactant molecules. Starting with the stochastic dynamics of the molecules, we derive a master-diffusion equation for the joint probability density of a mobile reactant and the number of bound substrate in a confined domain. We use the equation to calculate the fluctuations in the number of bound substrate molecules as a function of initial reactant distribution. A second model is presented based on a Markov description of the binding and unbinding and on the mean first passage time of a molecule to a small portion of the boundary. These models can be used for the description of noise due to gating of ionic channels by random binding and unbinding of ligands in biological sensor cells, such as olfactory cilia, photo-receptors, hair cells in the cochlea.

D. Holcman; Z. Schuss

2004-12-25

85

Assessing the importance of proton transfer reactions in DNA.  

PubMed

Although engineered by millions of years of evolution, the cellular machinery is not flawless, and errors regularly appear during DNA replication. The subsequent alteration of the stored genetic message results in a mutation and might be the starting point of important health disorders. The question therefore is what causes DNA mutations? All living organisms are constantly exposed to a number of external agents such as free radicals and to radiation, which may lead to induced mutations. There are also mutations happening without invoking the action of any exogenous element, the so-called spontaneous mutations. The former can be partially controlled by avoiding exposure to high-risk environments, while the latter are more intriguing because their origin is unclear and difficult to determine. As noted by Watson and Crick when they first discovered the DNA structure, the correct replication of DNA rests on the assumption that the base pairs remain in their most stable, canonical form. However, protons along the interbase hydrogen-bond network are not static entities. They can in fact interchange their positions in DNA bases through proton transfer (PT) reactions before strands unwind, giving rise to noncanonical structures defined as rare tautomers. The importance of these rare tautomers was also cleverly anticipated by Watson and Crick and some years later claimed by Löwdin to be a source of spontaneous mutations. In Watson and Crick's words: "It would be of interest to know the precise difference in free energy between the various tautomeric forms under physiological conditions." Unfortunately, rare tautomeric forms are very difficult to detect, so no direct and accurate free energy measure has been discerned. In contrast, theoretical chemistry is making good progress toward the quantification of PT reactions in DNA and their biological consequences. This Account touches upon the theoretical studies devoted to appraising the importance of rare tautomers as promoters of spontaneous mutations. We focus in particular on the crucial role played by the biological environment on DNA stability. It has now been demonstrated that valuable macroscopic predictions require not only highly accurate theories but also refined chemical models. Hybrid quantum mechanics/molecular mechanics (QM/MM) simulations performed on short but complete DNA sequence fragments emerge in this context as the most adequate tools. In addition, these methods can be used to quantify the effect of different external agents on the PT tautomeric equilibria and, eventually, to conveniently handle them. This is the case for the possible alteration of the naturally observed mutation rate by exposure to intense electric fields. Theoretical predictions envision in this respect promising applications of ultrashort electric pulses in medicine to selectively modify the mutated/canonical ratio in DNA. PMID:24849375

Jacquemin, Denis; Zúñiga, José; Requena, Alberto; Céron-Carrasco, José Pedro

2014-08-19

86

Computer simulation of chemical reactions in porous materials  

NASA Astrophysics Data System (ADS)

Understanding reactions in nanoporous materials from a purely experimental perspective is a difficult task. Measuring the chemical composition of a reacting system within a catalytic material is usually only accomplished through indirect methods, and it is usually impossible to distinguish between true chemical equilibrium and metastable states. In addition, measuring molecular orientation or distribution profiles within porous systems is not easily accomplished. However, molecular simulation techniques are well-suited to these challenges. With appropriate simulation techniques and realistic molecular models, it is possible to validate the dominant physical and chemical forces controlling nanoscale reactivity. Novel nanostructured catalysts and supports can be designed, optimized, and tested using high-performance computing and advanced modeling techniques in order to guide the search for next-generation catalysts---setting new targets for the materials synthesis community. We have simulated the conversion of several different equilibrium-limited reactions within microporous carbons and we find that the pore size, pore geometry, and surface chemistry are important factors for determining the reaction yield. The equilibrium-limited reactions that we have modeled include nitric oxide dimerization, ammonia synthesis, and the esterification of acetic acid, all of which show yield enhancements within microporous carbons. In conjunction with a yield enhancement of the esterification reaction, selective adsorption of ethyl acetate within carbon micropores demonstrates an efficient method for product recovery. Additionally, a new method has been developed for simulating reaction kinetics within porous materials and other heterogeneous environments. The validity of this technique is first demonstrated by reproducing the kinetics of hydrogen iodide decomposition in the gas phase, and then predictions are made within slit-shaped carbon pores and carbon nanotubes. The rate constant is found to increase by a factor of 47 in carbon nanotubes, as compared to the same reaction in the bulk gas phase. Overall, the results of these simulation studies demonstrate improvements in chemical reaction yield and chemical kinetics that are possible by understanding the nature of confined reactions, and applying this knowledge to catalyst design.

Turner, Christoffer Heath

87

Nonlinear magnetoacoustic wave propagation with chemical reactions  

NASA Astrophysics Data System (ADS)

The magnetoacoustic problem with an application to sound wave propagation through electrically conducting fluids such as the ocean in the Earth's magnetic field, liquid metals, or plasmas has been addressed taking into account several simultaneous chemical reactions. Using continuum balance equations for the total mass, linear momentum, energy; as well as Maxwell's electrodynamic equations, a nonlinear beam equation has been developed to generalize the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation for a fluid with linear viscosity but nonlinear and diffraction effects. Thermodynamic parameters are used and not tailored to only an adiabatic fluid case. The chemical kinetic equations build on a relaxing media approach presented, for example, by K. Naugolnukh and L. Ostrovsky [Nonlinear Wave Processes in Acoustics (Cambridge Univ. Press, Cambridge, 1998)] for a linearized single reaction and thermodynamic pressure equation of state. Approximations for large and small relaxation times and for magnetohydrodynamic parameters [Korsunskii, Sov. Phys. Acoust. 36 (1990)] are examined. Additionally, Cattaneo's equation for heat conduction and its generalization for a memory process rather than a Fourier's law are taken into account. It was introduced for the heat flux depends on the temperature gradient at an earlier time to generate heat pulses of finite speed.

Margulies, Timothy Scott

2002-11-01

88

Heat-Of-Reaction Chemical Heat Pumps--Possible Configurations  

E-print Network

" can mean a chemical reaction or sorption or mixing proc ess. Distinction between reaction and sorption processes is not necessary to convey concepts central to this paper, but the focus of this work is cycles with chemical reactions.) I f... supported by the U.S. Department of Energy, Director of Energy Research, Office of Industrial Programs, under DOE Contract No. 1l1':-AC07-76ID01570. HEAT-OF-REACTION CH.EMICAL HEAT PUMPS--POSSIBLE CONFIGUR.?\

Kirol, L. D.

89

Detailed Chemical Kinetic Reaction Mechanism for Biodiesel Components Methyl Stearate and Methyl Oleate  

SciTech Connect

New chemical kinetic reaction mechanisms are developed for two of the five major components of biodiesel fuel, methyl stearate and methyl oleate. The mechanisms are produced using existing reaction classes and rules for reaction rates, with additional reaction classes to describe other reactions unique to methyl ester species. Mechanism capabilities were examined by computing fuel/air autoignition delay times and comparing the results with more conventional hydrocarbon fuels for which experimental results are available. Additional comparisons were carried out with measured results taken from jet-stirred reactor experiments for rapeseed methyl ester fuels. In both sets of computational tests, methyl oleate was found to be slightly less reactive than methyl stearate, and an explanation of this observation is made showing that the double bond in methyl oleate inhibits certain low temperature chain branching reaction pathways important in methyl stearate. The resulting detailed chemical kinetic reaction mechanism includes more approximately 3500 chemical species and more than 17,000 chemical reactions.

Naik, C; Westbrook, C K; Herbinet, O; Pitz, W J; Mehl, M

2010-01-22

90

Atmospheric chemistry: Laboratory studies of kinetics of important reactions  

NASA Astrophysics Data System (ADS)

Experiments to measure the rate constants for some reactions of the atmospherically important nitrate radical (NO3) are described using the discharge-flow technique. The nitrate radical was monitored by optical absorption at lambda = 662 nm. The reactions of NO3 with some stable organic and inorganic substrates are reported. The temperature dependences of some of the rate constants were also determined (298 less than T less than 523 K). In most cases, computer simulation was used to extract the rate constant for the primary process because the time-dependent behavior of (NO3) was affected by secondary reactions of NO3 with products of the primary interaction. The Arrhenius parameter for the reactions of NO3 with CH3CH3, CH2CH2, CH3OH, CHCl3, and HCl were determined. The activation energies for the reactions studied between NO3 and some alkynes are presented along with the corresponding pre-exponential factors. Some reactions were studied at room temperature (298 plus or minus 2 K) only and the rate constants found (in units of cubic cm/molecule sec) are: buta-1,3-diene (1.8 x 10 (exp -13), isobutene (2.8 x 10 (exp -13), HBr (1.3 x 10 (exp -15) and hex-2-yne (3.0 x 10 (exp -14). Non-Arrhenius behavior was found in the reactions of NO3 with n-butane, isobutane and propene. The empirical variation of these rate constants with temperature is presented. The curvature of the Arrhenius plots is discussed in terms of (1) a temperature-dependent pre-exponential factor, and (2) the possibility that two competing channels, possessing differing activation energies, exist for each reaction. The atmospheric implications of these reactions are discussed with reference to the nighttime production of nitric acid and the importance of the these reactions as loss processes for NO3.

Smith, S. J.

91

Quantum theory of chemical reaction rates  

SciTech Connect

If one wishes to describe a chemical reaction at the most detailed level possible, i.e., its state-to-state differential scattering cross section, then it is necessary to solve the Schroedinger equation to obtain the S-matrix as a function of total energy E and total angular momentum J, in terms of which the cross sections can be calculated as given by equation (1) in the paper. All other physically observable attributes of the reaction can be derived from the cross sections. Often, in fact, one is primarily interested in the least detailed quantity which characterizes the reaction, namely its thermal rate constant, which is obtained by integrating Eq. (1) over all scattering angles, summing over all product quantum states, and Boltzmann-averaging over all initial quantum states of reactants. With the proper weighting factors, all of these averages are conveniently contained in the cumulative reaction probability (CRP), which is defined by equation (2) and in terms of which the thermal rate constant is given by equation (3). Thus, having carried out a full state-to-state scattering calculation to obtain the S-matrix, one can obtain the CRP from Eq. (2), and then rate constant from Eq. (3), but this seems like ``overkill``; i.e., if one only wants the rate constant, it would clearly be desirable to have a theory that allows one to calculate it, or the CRP, more directly than via Eq. (2), yet also correctly, i.e., without inherent approximations. Such a theory is the subject of this paper.

Miller, W.H. [Univ. of California, Berkeley, CA (United States). Dept. of Chemistry]|[Lawrence Berkeley Lab., CA (United States). Chemical Sciences Div.

1994-10-01

92

Chemical reaction fouling model for single-phase heat transfer  

SciTech Connect

A fouling model was developed on the premise that the chemical reaction for generation of precursor can take place in the bulk fluid, in the thermalboundary layer, or at the fluid/wall interface, depending upon the interactive effects of flu id dynamics, heat and mass transfer, and the controlling chemical reaction. The analysis was used to examine the experimental data for fouling deposition of polyperoxides produced by autoxidation of indene in kerosene. The effects of fluid and wall temperatures for two flow geometries were analyzed. The results showed that the relative effects of physical parameters on the fouling rate would differ for the three fouling mechanisms; therefore, it is important to identify the controlling mechanism in applying the closed-flow-loop data to industrial conditions.

Panchal, C.B. [Argonne National Lab., IL (United States); Watkinson, A.P. [British Columbia Univ., Vancouver, BC (Canada)

1993-08-01

93

Chemical reaction fouling model for single-phase heat transfer  

NASA Astrophysics Data System (ADS)

A fouling model was developed on the premise that the chemical reaction for generation of a precursor can take place in the bulk fluid, in the thermal boundary layer, or at the fluid/wall interface, depending upon the interactive effects of fluid dynamics, heat and mass transfer, and the controlling chemical reaction. The analysis was used to examine the experimental data for fouling deposition of polyperoxides produced by autoxidation of indene in kerosene. The effects of fluid and wall temperatures for two flow geometries were analyzed. The results showed that the relative effects of physical parameters on the fouling rate would differ for the three fouling mechanisms; therefore, it is important to identify the controlling mechanism in applying the closed-flow-loop data to industrial conditions.

Panchal, C. B.; Watkinson, A. P.

94

Silicon-based sleeve devices for chemical reactions  

DOEpatents

A silicon-based sleeve type chemical reaction chamber that combines heaters, such as doped polysilicon for heating, and bulk silicon for convection cooling. The reaction chamber combines a critical ratio of silicon and silicon nitride to the volume of material to be heated (e.g., a liquid) in order to provide uniform heating, yet low power requirements. The reaction chamber will also allow the introduction of a secondary tube (e.g., plastic) into the reaction sleeve that contains the reaction mixture thereby alleviating any potential materials incompatibility issues. The reaction chamber may be utilized in any chemical reaction system for synthesis or processing of organic, inorganic, or biochemical reactions, such as the polymerase chain reaction (PCR) and/or other DNA reactions, such as the ligase chain reaction, which are examples of a synthetic, thermal-cycling-based reaction. The reaction chamber may also be used in synthesis instruments, particularly those for DNA amplification and synthesis.

Northrup, M. Allen (Berkeley, CA); Mariella, Jr., Raymond P. (Danville, CA); Carrano, Anthony V. (Livermore, CA); Balch, Joseph W. (Livermore, CA)

1996-01-01

95

Silicon-based sleeve devices for chemical reactions  

DOEpatents

A silicon-based sleeve type chemical reaction chamber is described that combines heaters, such as doped polysilicon for heating, and bulk silicon for convection cooling. The reaction chamber combines a critical ratio of silicon and silicon nitride to the volume of material to be heated (e.g., a liquid) in order to provide uniform heating, yet low power requirements. The reaction chamber will also allow the introduction of a secondary tube (e.g., plastic) into the reaction sleeve that contains the reaction mixture thereby alleviating any potential materials incompatibility issues. The reaction chamber may be utilized in any chemical reaction system for synthesis or processing of organic, inorganic, or biochemical reactions, such as the polymerase chain reaction (PCR) and/or other DNA reactions, such as the ligase chain reaction, which are examples of a synthetic, thermal-cycling-based reaction. The reaction chamber may also be used in synthesis instruments, particularly those for DNA amplification and synthesis. 32 figs.

Northrup, M.A.; Mariella, R.P. Jr.; Carrano, A.V.; Balch, J.W.

1996-12-31

96

Astrobiology - the final frontier in chemical reaction dynamics  

NASA Astrophysics Data System (ADS)

Crossed-beam experiments on the reactions of cyano CN(X2?+) and ethinyl C2H(X2?+) radicals with the unsaturated hydrocarbons acetylene, ethylene, methylacetylene allene and benzene have been carried out under single-collision conditions to investigate synthetic routes to form nitriles, polyynes and substituted allenes in hydrocarbon-rich atmospheres of planets and their moons. All reactions were found to proceed without an entrance barrier, to have exit barriers well below the energy of the reactant molecules and to be strongly exothermic. The predominant identification of the radical versus atomic hydrogen exchange channel makes these reactions compelling candidates for the formation of complex organic chemicals - precursors to biologically important amino acids - in Solar system environments and in the interstellar medium.

Kaiser, Ralf I.; Balucani, Nadia

2002-01-01

97

Thermodynamic performance for a chemical reactions model  

NASA Astrophysics Data System (ADS)

This paper presents the analysis efficiency of a chemical reaction model of four states, such that their activated states can occur at any point (fixed but arbitrary) of the transition from one state to another. This mechanism operates under a single heat reservoir temperature, unlike the internal combustion engines where there are two thermal sources. Different efficiencies are compared to this model, which operate at different optimum engine regimes. Thus, some analytical methods are used to give an approximate expression, facilitating the comparison between them. Finally, the result is compared with that obtained by other authors considered a general model of an isothermal molecular machine. Taking into account the above, the results seems to follow a similar behaviour for all the optimized engines, which resemble that observed in the case of heat engine efficiencies.

Gonzalez-Narvaez, R. E.; Sánchez-Salas, N.; Chimal-Eguía, J. C.

2015-01-01

98

Some chemical and mineralogical considerations important for understanding leachate chemistry  

SciTech Connect

Both the raw shale material and the retorting process parameters are influential in determining the subsequent behavior of the spent shale solids with respect to leaching and/or environmental weathering. The process parameters define the mineral reactions that occur to form the mineral assemblage in the spent shale, while the major and trace element residences and mobilities from the raw shale determine the extent of incorporation of these elements in the spent shale matrix and ultimately the composition of leachate generated by the spent shale solid: water interaction. In order to understand leachate compositions, it is necessary to determine this water: solid interaction, but the solid wastes being considered are a dependent function of the raw shale material and the process parameters. Thus, in order to understand the chemical principles operative in leachate generation, it is necessary to elucidate the interplay of the raw material and the process parameters in the formation of the waste and then the interaction of the waste form with water. The leachate and the solid waste are dependent variables, while the raw shale and the process parameters are independent variables. These considerations are illustrated by results of chemical characterization and experimental studies of field generated spent shales. Results from field generated materials are used to describe important considerations relative to the understanding of leachate chemistry.

Peterson, E.J.; Wagner, P.

1982-01-01

99

Plasmon-assisted chemical reactions revealed by high-vacuum tip-enhanced Raman spectroscopy  

NASA Astrophysics Data System (ADS)

Tip-enhanced Raman spectroscopy (TERS) is the technique that combines the nanoscale spatial resolution of a scanning probe microscope and the highly sensitive Raman spectroscopy enhanced by the surface plasmons. It is suitable for chemical analysis at nanometer scale. Recently, TERS exhibited powerful potential in analyzing the chemical reactions at nanoscale. The high sensitivity and spatial resolution of TERS enable us to learn the reaction processes more clearly. More importantly, the chemical reaction in TERS is assisted by surface plasmons, which provides us an optical method to manipulate the chemical reactions at nanoscale. Here using our home-built high-vacuum tip-enhanced Raman spectroscopy (HV-TERS) setup, we successfully observed the plasmon-assisted molecule dimerization and dissociation reactions. In HV-TERS system, under laser illumination, 4-nitrobenzenethiol (4NBT) molecules can be dimerized to p,p'-dimercaptoazobenzene (DMAB), and dissociation reaction occurs for malachite green (MG) molecules. Using our HV-TERS setup, the dynamic processes of the reactions are clearly revealed. The chemical reactions can be manipulated by controlling the plasmon intensity through changing the power of the incident laser, the tunneling current and the bias voltage. We also investigated the role of plasmonic thermal effect in the reactions by measuring both the Stokes and anti- Stokes Raman peaks. Our findings extend the applications of TERS, which can help to study the chemical reactions and understand the dynamic processes at single molecular level, and even design molecules by the plasmon-assisted chemical reactions.

Lu, Shuaicheng; Sheng, Shaoxiang; Zhang, Zhenglong; Xu, Hongxing; Zheng, Hairong

2014-08-01

100

GREEN CHEMICAL SYNTHESIS THROUGH CATALYSIS AND ALTERNATE REACTION CONDITIONS  

EPA Science Inventory

Green chemical synthesis through catalysis and alternate reaction conditions Encompassing green chemistry techniques and methodologies, we have initiated several projects at the National Risk Management Research laboratory that focus on the design and development of chemic...

101

Crossed molecular beam studies of atmospheric chemical reaction dynamics  

SciTech Connect

The dynamics of several elementary chemical reactions that are important in atmospheric chemistry are investigated. The reactive scattering of ground state chlorine or bromine atoms with ozone molecules and ground state chlorine atoms with nitrogen dioxide molecules is studied using a crossed molecular beams apparatus with a rotatable mass spectrometer detector. The Cl + O{sub 3} {yields} ClO + O{sub 2} reaction has been studied at four collision energies ranging from 6 kcal/mole to 32 kcal/mole. The derived product center-of-mass angular and translational energy distributions show that the reaction has a direct reaction mechanism and that there is a strong repulsion on the exit channel. The ClO product is sideways and forward scattered with respect to the Cl atom, and the translational energy release is large. The Cl atom is most likely to attack the terminal oxygen atom of the ozone molecule. The Br + O{sub 3} {yields} ClO + O{sub 2} reaction has been studied at five collision energies ranging from 5 kcal/mole to 26 kcal/mole. The derived product center-of-mass angular and translational energy distributions are quite similar to those in the Cl + O{sub 3} reaction. The Br + O{sub 3} reaction has a direct reaction mechanism similar to that of the Cl + O{sub 3} reaction. The electronic structure of the ozone molecule seems to play the central role in determining the reaction mechanism in atomic radical reactions with the ozone molecule. The Cl + NO{sub 2} {yields} ClO + NO reaction has been studied at three collision energies ranging from 10.6 kcal/mole to 22.4 kcal/mole. The center-of-mass angular distribution has some forward-backward symmetry, and the product translational energy release is quite large. The reaction proceeds through a short-lived complex whose lifetime is less than one rotational period. The experimental results seem to show that the Cl atom mainly attacks the oxygen atom instead of the nitrogen atom of the NO{sub 2} molecule.

Zhang, Jingsong

1993-04-01

102

Thermodynamics of industrially-important, enzyme-catalyzed reactions  

Microsoft Academic Search

The thermodynamics of 10 industrially-important, enzyme-catalyzed reactions are examined. The reactions discussed are: the\\u000a conversions of penicillin G to 6-amino-penicillinic acid using the enzyme penicillin acylase; starch to glucose using amylases;\\u000a glucose to fructose using glucose (xylose) isomerase; cellulose to glucose using cellulase; fumaric acid and ammonia to l-aspartic\\u000a acid using l-aspartase; transcinnamic acid and ammonia to l-phenylalanine using l-phenylalanine

Yadu B. Tewari

1990-01-01

103

Direct Monte Carlo simulation of chemical reaction systems: Simple bimolecular reactions  

E-print Network

Direct Monte Carlo simulation of chemical reaction systems: Simple bimolecular reactions Shannon D and understanding the behavior of gas phase chemical reaction systems. This Monte Carlo method, originated by Bird useful, and the gas dynamics of many systems is more easily predicted and understood by using Monte Carlo

Anderson, James B.

104

Heterogeneous chemical reactions: Preparation of monodisperse latexes  

NASA Technical Reports Server (NTRS)

It is demonstrated that a photoinitiated emulsion polymerization can be carried out to a significant conversion in a SPAR rocket prototype polymerization vessel within the six minutes allowed for the experiment. The percentage of conversion was determined by both dilatometry and gravimetric methods with good agreement. The experimental results lead to the following conclusions: (1) emulsion polymerizations can be carried out to conversions as high as 75%, using a stable micellized styrene-SLS system plus photoinitiator; (2) dilatometry can be used to accurately determine both the rate and conversion of polymerization; (3) thermal expansion due to the light source and heat of reaction is small and can be corrected for if necessary; (4) although seeded emulsion polymerizations are unfavorable in photoinitiation, as opposed to chemical initiation, polymerizations can be carried out to at least 15% conversion using 7940A seed particles, with 0.05% solids; and (5) photoinitiation should be used to initiate polymerization in the SPAR rocket experiments because of the mechanical simplicity of the experiment.

Vanderhoff, J. W.; Micale, F. J.; El-Aasser, M. S.; Sterk, A. A.; Bethke, G. W.

1977-01-01

105

Thermophoretic transport in impinging flows including chemical reactions  

NASA Astrophysics Data System (ADS)

The research constitutes a fundamental study of transport phenomena that has applications to the fabrication of optical fibers. The flow, heat and mass transfer with chemical reactions, and thermophoretic transport of silica particles are studied. The governing equations have been formulated and solved to determine the velocity, temperature and species concentrations, and the effects of buoyancy, variable properties, chemical reactions, and thermophoretic transport on deposition uniformity and efficiency. A path-line/stream-line approach for determining variable particle concentration stagnation deposition with thermophoretic transport is introduced. The research is of fundamental interest and has applications to optical fiber fabrication. A review of the thermophoresis phenomena is provided including a recommended treatment for the thermophoretic coefficient, K. A discussion of the related chemical reactions is given. A study of a H2-N 2 non-premixed jet flame is performed to appraise the proposed H2 oxidation reaction rate and heat release rate. The present results of the velocity and temperature distributions are in very good agreement with the published experimental data. A study of silica (SiO2) particle transport in a free jet system is carried out. The gas entrainment and the thermophoretic transport are two important parameters that determine the particle transport. Both non-reacting and reacting jet flows are studied. The effects of inclination of jet flows on particle transport are also studied. A path-line/stream-line approach for determining deposition with thermophoretic transport in stagnation flow is introduced. This approach allows a variable particle concentration inlet boundary condition to be used. A closed form solution is achieved using this approach. Numerical studies of deposition on a cylindrical preform for both non-reacting and reacting jets are also investigated. A study of a small scale fiber fabrication is made with a cylindrical preform and two burners. Of special interest are the effects of preform temperature and preform thermal conductivity on the deposition efficiency.

Hsu, Frank Kuan-Chao

106

Single-collision studies of energy transfer and chemical reaction  

SciTech Connect

The research focus in this group is state-to-state dynamics of reaction and energy transfer in collisions of free radicals such as H, OH, and CH{sub 3} with H{sub 2}, alkanes, alcohols and other hydrogen-containing molecules. The motivation for the work is the desire to provide a detailed understanding of the chemical dynamics of prototype reactions that are important in the production and utilization of energy sources, most importantly in combustion. The work is primarily experimental, but with an important and growing theoretical/computational component. The focus of this research program is now on reactions in which at least one of the reactants and one of the products is polyatomic. The objective is to determine how the high dimensionality of the reactants and products differentiates such reactions from atom + diatom reactions of the same kinematics and energetics. The experiments use highly time-resolved laser spectroscopic methods to prepare reactant states and analyze the states of the products on a single-collision time scale. The primary spectroscopic tool for product state analysis is coherent anti-Stokes Raman scattering (CARS) spectroscopy. CARS is used because of its generality and because the extraction of quantum state populations from CARS spectra is straightforward. The combination of the generality and easy analysis of CARS makes possible absolute cross section measurements (both state-to-state and total), a particularly valuable capability for characterizing reactive and inelastic collisions. Reactant free radicals are produced by laser photolysis of appropriate precursors. For reactant vibrational excitation stimulated Raman techniques are being developed and implemented.

Valentini, J.J. [Columbia Univ., New York, NY (United States)

1993-12-01

107

Students’ Microscopic, Macroscopic, and Symbolic Representations of Chemical Reactions  

Microsoft Academic Search

This study examined the mental representations of chemical reactions used by six students (three male, three female) who achieved above-average grades in a college freshman chemistry class at a large midwestern university. The representations expressed by the students in structured interviews were categorized as microscopic, macroscopic, or symbolic representations of chemical reactions. The study revealed that the participants did make

Michael E. Hinton; Mary B. Nakhleh

1999-01-01

108

On the rate of relativistic surface chemical reactions.  

PubMed

On the basis of special relativity and the classical theory of chemical reaction rates it is shown how the surface chemical reaction rates vary as v --> c, where v is the velocity of the object under study and c is the velocity of light. PMID:15178286

Veitsman, E V

2004-07-15

109

Investigating Factors Influencing Rates of Chemical Reactions  

NSDL National Science Digital Library

This activity is a lab investigation in which students observe the rate of generation of hydrogen gas from a reaction, and then modify the procedure to compare another variable affecting the rate of this reaction.

Paula Derickson

110

CHEMSIMUL --a tool for simulating chemical reaction systems Peter Kirkegaard, Erling Bjergbakke, and Frank Markert  

E-print Network

CHEMSIMUL -- a tool for simulating chemical reaction systems Peter Kirkegaard, Erling Bjergbakke difficult to study elementary chemical reactions without interference from simul- taneous side reactions simulator of chemical kinetics with the following main compo- nents: - Module for input of reaction

111

Mesoscale simulations of shockwave energy dissipation via chemical reactions  

NASA Astrophysics Data System (ADS)

We use a particle-based mesoscale model that incorporates chemical reactions at a coarse-grained level to study the response of materials that undergo volume-reducing chemical reactions under shockwave-loading conditions. We find that such chemical reactions can attenuate the shockwave and characterize how the parameters of the chemical model affect this behavior. The simulations show that the magnitude of the volume collapse and velocity at which the chemistry propagates are critical to weaken the shock, whereas the energetics in the reactions play only a minor role. Shock loading results in transient states where the material is away from local equilibrium and, interestingly, chemical reactions can nucleate under such non-equilibrium states. Thus, the timescales for equilibration between the various degrees of freedom in the material affect the shock-induced chemistry and its ability to attenuate the propagating shock.

Antillon, Edwin; Strachan, Alejandro

2015-02-01

112

Mechano-chemical coupling in Belousov-Zhabotinskii reactions  

NASA Astrophysics Data System (ADS)

Mechano-chemical coupling has been recently recognised as an important effect in various systems as chemical reactivity can be controlled through an applied mechanical loading. Namely, Belousov-Zhabotinskii reactions in polymer gels exhibit self-sustained oscillations and have been identified to be reasonably controllable and definable to the extent that they can be harnessed to perform mechanical work at specific locations. In this paper, we use our theoretical work of nonlinear mechano-chemical coupling and investigate the possibility of providing an explanation of phenomena found in experimental research by means of this theory. We show that mechanotransduction occurs as a response to both static and dynamic mechanical stimulation, e.g., volume change and its rate, as observed experimentally and discuss the difference of their effects on oscillations. Plausible values of the quasi-stoichiometric parameter f of Oregonator model are estimated together with its dependence on mechanical stimulation. An increase in static loading, e.g., pressure, is predicted to have stimulatory effect whereas dynamic loading, e.g., rate of volume change, is predicted to be stimulatory only up to a certain threshold. Further, we offer a physically consistent explanation of the observed phenomena why some Belousov-Zhabotinskii gels require an additional mechanical stimulation to show emergence of oscillation or why "revival" of oscillations in Belousov-Zhabotinskii reactions is possible together with indications for further experimental setups.

Klika, Václav; Grmela, Miroslav

2014-03-01

113

Negative Temperature Coefficient in Chemical Reactions  

Microsoft Academic Search

A systematic analysis of reactions whose rate decreases with increase of temperature is presented. The possibility of a negative temperature coefficient in the elementary reactions is examined from the standpoint of the transition state theory and of collision theory. The mechanisms of complex reactions in which the temperature dependence of the rate is anomalous are discussed, and possible reasons for

I A Leenson; Gleb B Sergeev

1984-01-01

114

LIGAND: database of chemical compounds and reactions in biological pathways  

Microsoft Academic Search

LIGAND is a composite database comprising three sections: COMPOUND for the information about metabolites and other chemical compounds, REACTION for the collection of substrate-product relations representing metabolic and other reactions, and ENZYME for the information about enzyme molecules. The current release (as of September 7, 2001) includes 7298 compounds, 5166 reactions and 3829 enzymes. In addition to the keyword search

Susumu Goto; Yasushi Okuno; Masahiro Hattori; Takaaki Nishioka; Minoru Kanehisa

2002-01-01

115

Semiclassical methods in chemical reaction dynamics  

SciTech Connect

Semiclassical approximations, simple as well as rigorous, are formulated in order to be able to describe gas phase chemical reactions in large systems. We formulate a simple but accurate semiclassical model for incorporating multidimensional tunneling in classical trajectory simulations. This model is based on the existence of locally conserved actions around the saddle point region on a multidimensional potential energy surface. Using classical perturbation theory and monitoring the imaginary action as a function of time along a classical trajectory we calculate state-specific unimolecular decay rates for a model two dimensional potential with coupling. Results are in good comparison with exact quantum results for the potential over a wide range of coupling constants. We propose a new semiclassical hybrid method to calculate state-to-state S-matrix elements for bimolecular reactive scattering. The accuracy of the Van Vleck-Gutzwiller propagator and the short time dynamics of the system make this method self-consistent and accurate. We also go beyond the stationary phase approximation by doing the resulting integrals exactly (numerically). As a result, classically forbidden probabilties are calculated with purely real time classical trajectories within this approach. Application to the one dimensional Eckart barrier demonstrates the accuracy of this approach. Successful application of the semiclassical hybrid approach to collinear reactive scattering is prevented by the phenomenon of chaotic scattering. The modified Filinov approach to evaluating the integrals is discussed, but application to collinear systems requires a more careful analysis. In three and higher dimensional scattering systems, chaotic scattering is suppressed and hence the accuracy and usefulness of the semiclassical method should be tested for such systems.

Keshavamurthy, S.

1994-12-01

116

Incidents of chemical reactions in cell equipment  

SciTech Connect

Strongly exothermic reactions can occur between equipment structural components and process gases under certain accident conditions in the diffusion enrichment cascades. This paper describes the conditions required for initiation of these reactions, and describes the range of such reactions experienced over nearly 50 years of equipment operation in the US uranium enrichment program. Factors are cited which can promote or limit the destructive extent of these reactions, and process operations are described which are designed to control the reactions to minimize equipment damage, downtime, and the possibility of material releases.

Baldwin, N.M.; Barlow, C.R. [Uranium Enrichment Organization, Oak Ridge, TN (United States)

1991-12-31

117

Atmospheric Chemistry: Laboratory Studies of Kinetics of Important Reactions.  

NASA Astrophysics Data System (ADS)

Available from UMI in association with The British Library. Requires signed TDF. This thesis describes the experiments to measure the rate constants for some reactions of the atmospherically important nitrate radical (NO_3) using the discharge-flow technique. The nitrate radical was monitored by optical absorption at lambda = 662 nm. The reactions of NO_3 with some stable organic and inorganic substrates are reported. The temperature dependences of some of the rate constants have also been determined (298 < T < 523 K). In most cases, computer simulation was used to extract the rate constant for the primary process because the time-dependent behaviour of (NO_3) was affected by secondary reactions of NO_3 with products of the primary interaction. The Arrhenius parameter in parentheses (E _{rm a}/kJ mol^ {-1}, A/cm^3 molecule ^{-1}s^ {-1} respectively) for the following reactions have been determined: ethane (37, 6.7 times 10^{-12}), ethylene (25.8, 6.3 times 10^ {-12}), CH_3OH (21.3, 1.2 times 10^ {-12}), CHCiota_3 (23.4, 8.6 times 10 ^{-13}) and HCl (27.7, 4 times 10^{-12}). The activation energies for the reactions studied between NO_3 and some alkynes are represented well by the value 25 +/- 3 kJ mol^{-1} and the corresponding pre-exponential factors (expressed as ln(10 ^{13}A/cm^3 molecule^{-1}s ^{-1}) are as follows: C_2H_2 (1.6 +/- 1.4), C_3H _4 (5.0 +/- 1.4), 1-C_4H_6 (5.8 +/- 1.0), 1-C_5 H_8 (5.7 +/- 0.6) and 1-C_6H _{10} (4.5 +/- 0.4). Some reactions were studied at room temperature _3(298 +/- 2 K) only and the rate constants found (in units of cm ^3 molecule^{ -1}s^{-1}) are: buta-1,3-diene (1.8 times 10 ^{-13}), isobutene (2.8 times 10^{-13 }), HBr (1.3 times 10 ^{-15}) and hex-2-yne (3.0 times 10^{-14 }). Non-Arrhenius behaviour was found in the reactions of NO_3 with n-butane, isobutane and propene. The empirical variation of these rate constants with temperature is well represented by the three parameter expressions:. k(T) = 1.2 times 10 ^{-46}T^{11.4 }exp(-1131/T) for n-butane; k(T) = 1.6 times 10^ {-49}T^{12.3} exp(-1828/T) for isobutane; and for propene, k(T) = 2.4 times 10 ^{-19}T^ {2.25}exp(-700/T). The curvature of the Arrhenius plots is discussed in terms of (a) a temperature-dependent pre-exponential factor and (b) the possibility that two competing channels, possessing differing activation energies, exist for each reaction. The atmospheric implications of these reactions are discussed with reference to the nigh-time production of nitric acid and the importance of the these reactions as loss processes for NO_3.

Smith, S. J.

118

Flow Tube Studies of Gas Phase Chemical Processes of Atmospheric Importance  

NASA Technical Reports Server (NTRS)

The objective of this project is to conduct measurements of elementary reaction rate constants and photochemistry parameters for processes of importance in the atmosphere. These measurements are being carried out under temperature and pressure conditions covering those applicable to the stratosphere and upper troposphere, using the chemical ionization mass spectrometry turbulent flow technique developed in our laboratory.

Molina, Mario J.

1997-01-01

119

Chemical Demonstrations with Consumer Chemicals: The Black and White Reaction.  

ERIC Educational Resources Information Center

Describes a dramatic chemical demonstration in which chemicals that are black and white combine to produce a colorless liquid. Reactants include tincture of iodine, bleach, white vinegar, Epsom salt, vitamin C tablets, and liquid laundry starch. (DDR)

Wright, Stephen W.

2002-01-01

120

A Case Study in Chemical Kinetics: The OH + CO Reaction.  

ERIC Educational Resources Information Center

Reviews some important properties of the bimolecular reaction between the hydroxyl radical and carbon monoxide. Investigates the kinetics of the reaction, the temperature and pressure dependence of the rate constant, the state-to-state dynamics of the reaction, and the reverse reaction. (MVL)

Weston, Ralph E., Jr.

1988-01-01

121

Laboratory Studies of Homogeneous and Heterogeneous Chemical Processes of Importance in the Upper Atmosphere  

NASA Technical Reports Server (NTRS)

The objective of this study was to conduct measurements of chemical kinetics parameters for reactions of importance in the stratosphere and upper troposphere, and to study the interaction of trace gases with ice surfaces in order to elucidate the mechanism of heterogeneous chlorine activation processes, using both a theoretical and an experimental approach. The measurements were carried out under temperature and pressure conditions covering those applicable to the stratosphere and upper troposphere. The main experimental technique employed was turbulent flow-chemical ionization mass spectrometry, which is particularly well suited for investigations of radical-radical reactions.

Molina, Mario J.

2003-01-01

122

Chemical Looping Combustion Reactions and Systems  

SciTech Connect

Chemical Looping Combustion (CLC) is one promising fuel-combustion technology, which can facilitate economic CO2 capture in coal-fired power plants. It employs the oxidation/reduction characteristics of a metal, or oxygen carrier, and its oxide, the oxidizing gas (typically air) and the fuel source may be kept separate. This work focused on two classes of oxygen carrier, one that merely undergoes a change in oxidation state, such as Fe3O4/Fe2O3 and one that is converted from its higher to its lower oxidation state by the release of oxygen on heating, i.e., CuO/Cu2O. This topical report discusses the results of four complementary efforts: (1) the development of process and economic models to optimize important design considerations, such as oxygen carrier circulation rate, temperature, residence time; (2) the development of high-performance simulation capabilities for fluidized beds and the collection, parameter identification, and preliminary verification/uncertainty quantification (3) the exploration of operating characteristics in the laboratory-scale bubbling bed reactor, with a focus on the oxygen carrier performance, including reactivity, oxygen carrying capacity, attrition resistance, resistance to deactivation, cost and availability (4) the identification of mechanisms and rates for the copper, cuprous oxide, and cupric oxide system using thermogravimetric analysis.

Sarofim, Adel; Lighty, JoAnn; Smith, Philip; Whitty, Kevin; Eyring, Edward; Sahir, Asad; Alvarez, Milo; Hradisky, Michael; Clayton, Chris; Konya, Gabor; Baracki, Richard; Kelly, Kerry

2011-07-01

123

The Activated Complex in Chemical Reactions  

Microsoft Academic Search

The calculation of absolute reaction rates is formulated in terms of quantities which are available from the potential surfaces which can be constructed at the present time. The probability of the activated state is calculated using ordinary statistical mechanics. This probability multiplied by the rate of decomposition gives the specific rate of reaction. The occurrence of quantized vibrations in the

Henry Eyring

1935-01-01

124

Developing Secondary Students' Conceptions of Chemical Reactions: The Introduction of Chemical Equilibrium.  

ERIC Educational Resources Information Center

Describes an empirical study concerning the introduction of the concept of chemical equilibrium in chemistry classrooms in a way which challenges students' initial conceptions of chemical reactions. Contains 23 references. (DDR)

Van Driel, Jan H.; De Vos, Wobbe; Verloop, Nico; Dekkers, Hetty

1998-01-01

125

Morphological changes of amphiphilic molecular assemblies induced by chemical reaction  

E-print Network

Shape transformations of amphiphilic molecular assemblies induced by chemical reaction are studied using coarse-grained molecular simulations. A binding reaction between hydrophilic and hydrophobic molecules is considered. It is found that the reaction induces transformation of an oil droplet to a tubular vesicle via bicelles and vesicles with discoidal arms. The discoidal arms close into vesicles, which are subsequently fused into the tubular vesicle. Under the chemical reaction, the bicelle-to-vesicle transition occurs at smaller sizes than in the absence of the hydrophobic molecules. It is revealed that the enhancement of this transition is due to embedded hydrophobic particles that reduce the membrane bending rigidity.

Koh M. Nakagawa; Hiroshi Noguchi

2014-11-21

126

Non-equilibrium effects in high temperature chemical reactions  

NASA Technical Reports Server (NTRS)

Reaction rate data were collected for chemical reactions occurring at high temperatures during reentry of space vehicles. The principle of detailed balancing is used in modeling kinetics of chemical reactions at high temperatures. Although this principle does not hold for certain transient or incubation times in the initial phase of the reaction, it does seem to be valid for the rates of internal energy transitions that occur within molecules and atoms. That is, for every rate of transition within the internal energy states of atoms or molecules, there is an inverse rate that is related through an equilibrium expression involving the energy difference of the transition.

Johnson, Richard E.

1987-01-01

127

4.0 Application of Chemical Reaction Models Computerized chemical reaction models based on thermodynamic principles may be used to calculate  

E-print Network

4.0 Application of Chemical Reaction Models Computerized chemical reaction models based. The capabilities of a chemical reaction model depend on the models incorporated into its computer code is an equilibrium chemical reaction model. It was developed with EPA funding by originally combining

128

The Mystery Reaction: A Lesson on Chemical Reactions  

NSDL National Science Digital Library

This teaching resource was developed by a K-12 science teacher in the American Physiologycal Society's 2006 Frontiers in Physiology Program. For more information on this program, please visit www.frontiersinphys.org. The purpose of this lesson is to design an investigation and conduct an experiment that will allow students to explore the differences between physical and chemical changes. In this investigation, they are given the opportunity to develop a list of evidence for determining whether or not a chemical change has occurred.

Tonya Williams (Kelly Miller Middle School)

2006-08-01

129

Organic chemical reactions in supercritical water  

Microsoft Academic Search

Water near or above its critical point (374 C, 218 atm) is attracting increased attention as a medium for organic chemistry. Most of this new attention is driven by the search for more green or environmentally benign chemical processes. Using near-critical or supercritical water (SCW) instead of organic solvents in chemical processes offers environmental advantages and may lead to pollution

Phillip E. Savage

1999-01-01

130

FACILITATED CHEMICAL SYNTHESIS UNDER ALTERNATE REACTION CONDITIONS  

EPA Science Inventory

The chemical research in the late 1990's witnessed a paradigm shift towards "environmentally-friendly chemistry" more popularly known as "green chemistry" due to the increasing environmental concerns and legislative requirements to curb the release of chemical waste into the atmo...

131

Kinetics of Chemical Reactions in Flames  

NASA Technical Reports Server (NTRS)

In part I of the paper the theory of flame propagation is developed along the lines followed by Frank-Kamenetsky and one of the writers. The development of chain processes in flames is considered. A basis is given for the application of the method of stationary concentrations to reactions in flames; reactions with branching chains are analyzed. The case of a diffusion coefficient different from the coefficient of temperature conductivity is considered.

Zeldovich, Y.; Semenov, N.

1946-01-01

132

Application of Reversible Chemical Reactions for Temperature Amplification  

E-print Network

temperature thermal energy, mechanical and absorption type heat pumps have been proposed and developed so far. This paper addresses itself to the concept of a heat reaction chemical heat pump (HRCHP). The HRCHP concept is aimed to upgrade low temperature...

Ally, M. R.; Rebello, W. J.; Suciu, D. F.

133

29. NORTHWEST VIEW OF BOILER FEEDWATER CHEMICAL REACTION TANKS, WITH ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

29. NORTHWEST VIEW OF BOILER FEEDWATER CHEMICAL REACTION TANKS, WITH FORMER GENERAL OFFICE BUILDING IN BACKGROUND. - U.S. Steel Duquesne Works, Fuel & Utilities Plant, Along Monongahela River, Duquesne, Allegheny County, PA

134

Log-domain circuit models of chemical reactions  

E-print Network

We exploit the detailed similarities between electronics and chemistry to develop efficient, scalable bipolar or subthreshold log-domain circuits that are dynamically equivalent to networks of chemical reactions. Our ...

Mandal, Soumyajit

135

CHEMICAL REACTIONS SIMULATED BY GROUND-WATER-QUALITY MODELS.  

USGS Publications Warehouse

Recent literature concerning the modeling of chemical reactions during transport in ground water is examined with emphasis on sorption reactions. The theory of transport and reactions in porous media has been well documented. Numerous equations have been developed from this theory, to provide both continuous and sequential or multistep models, with the water phase considered for both mobile and immobile phases. Chemical reactions can be either equilibrium or non-equilibrium, and can be quantified in linear or non-linear mathematical forms. Non-equilibrium reactions can be separated into kinetic and diffusional rate-limiting mechanisms. Solutions to the equations are available by either analytical expressions or numerical techniques. Saturated and unsaturated batch, column, and field studies are discussed with one-dimensional, laboratory-column experiments predominating. A summary table is presented that references the various kinds of models studied and their applications in predicting chemical concentrations in ground waters.

Grove, David B.; Stollenwerk, Kenneth G.

1987-01-01

136

The Paternò-Büchi reaction: importance of triplet states in the excited-state reaction pathway.  

PubMed

The Paternò-Büchi (PB) reaction between an excited carbonyl compound and an alkene has been widely studied, but so far little is known about the excited-state dynamics of the reaction. In this investigation, we used a compound in which a formyl and a vinyl group are attached to a [2.2]paracyclophane in order to obtain a model system in pre-reactive conformation for the PB reaction. We studied the excited-state dynamics of the isolated molecule in a molecular beam using femtosecond time-resolved photoelectron spectroscopy and ab initio calculations. The results show that inter-system crossing within two picoseconds competes efficiently with the reaction in the singlet manifold. Thus, the PB reaction in this model system takes place in the triplet state on a time scale of nanoseconds. This result stresses the importance of triplet states in the excited-state pathway of the PB reaction involving aromatic carbonyl compounds, even in situations in which the reacting moieties are in immediate vicinity. PMID:22614254

Brogaard, Rasmus Y; Schalk, Oliver; Boguslavskiy, Andrey E; Enright, Gary D; Hopf, Henning; Raev, Vitaly; Tarcoveanu, Eliza; Sølling, Theis I; Stolow, Albert

2012-05-21

137

Advanced Chemical Heat Pumps Using Liquid-Vapor Reactions  

E-print Network

-vapor chemical reactions. . ~ Heat pumps using liquid-vapor reactions f (items 1 and 5, above) are the subjects of this paper. Configurations discussed are: r , ! 1. Electric drive 2. Temperature amplifier (TA) using Rankine heat engine and liquid... devices. The heat amplifier and reaction-reaction cycles are less promising. LIQUID-VAPOR EQUILIBRIUM Consider a general reaction of the form A --) B + C occurring in the two phase region. Gibb's phase rule for a reactive system is 4 F = N - R + 2...

Kirol, L.

138

Is the simplest chemical reaction really so simple?  

PubMed Central

Modern computational methods have become so powerful for predicting the outcome for the H + H2 ? H2 + H bimolecular exchange reaction that it might seem further experiments are not needed. Nevertheless, experiments have led the way to cause theorists to look more deeply into this simplest of all chemical reactions. The findings are less simple. PMID:24367084

Jankunas, Justin; Sneha, Mahima; Zare, Richard N.; Bouakline, Foudhil; Althorpe, Stuart C.; Herráez-Aguilar, Diego; Aoiz, F. Javier

2014-01-01

139

Understanding Chemical Reaction Kinetics and Equilibrium with Interlocking Building Blocks  

ERIC Educational Resources Information Center

Chemical reaction kinetics and equilibrium are essential core concepts of chemistry but are challenging topics for many students, both at the high school and undergraduate university level. Visualization at the molecular level is valuable to aid understanding of reaction kinetics and equilibrium. This activity provides a discovery-based method to…

Cloonan, Carrie A.; Nichol, Carolyn A.; Hutchinson, John S.

2011-01-01

140

Cu-free click cycloaddition reactions in chemical biology†  

PubMed Central

Bioorthogonal chemical reactions are paving the way for new innovations in biology. These reactions possess extreme selectivity and biocompatibility, such that their participating reagents can form covalent bonds within richly functionalized biological systems—in some cases, living organisms. This tutorial review will summarize the history of this emerging field, as well as recent progress in the development and application of bioorthogonal copper-free click cycloaddition reactions. PMID:20349533

Jewett, John C.

2010-01-01

141

Elementary reaction modeling of solid oxide electrolysis cells: Main zones for heterogeneous chemical/electrochemical reactions  

NASA Astrophysics Data System (ADS)

A theoretical model of solid oxide electrolysis cells considering the heterogeneous elementary reactions, electrochemical reactions and the transport process of mass and charge is applied to study the relative performance of H2O electrolysis, CO2 electrolysis and CO2/H2O co-electrolysis and the competitive behavior of heterogeneous chemical and electrochemical reactions. In cathode, heterogeneous chemical reactions exist near the outside surface and the electrochemical reactions occur near the electrolyte. According to the mathematical analysis, the mass transfer flux D ?c determines the main zone size of heterogeneous chemical reactions, while the charge transfer flux ? ?V determines the other one. When the zone size of heterogeneous chemistry is enlarged, more CO2 could react through heterogeneous chemical pathway, and polarization curves of CO2/H2O co-electrolysis could be prone to H2O electrolysis. Meanwhile, when the zone size of electrochemistry is enlarged, more CO2 could react through electrochemical pathway, and polarization curves of CO2/H2O co-electrolysis could be prone to CO2 electrolysis. The relative polarization curves, the ratio of CO2 participating in electrolysis and heterogeneous chemical reactions, the mass and charge transfer flux and heterogeneous chemical/electrochemical reaction main zones are simulated to study the effects of cathode material characteristics (porosity, particle diameter and ionic conductivity) and operating conditions (gas composition and temperature).

Li, Wenying; Shi, Yixiang; Luo, Yu; Cai, Ningsheng

2015-01-01

142

THE JOURNAL OF CHEMICAL PHYSICS 139, 165104 (2013) Exploring chemical reaction mechanisms through harmonic Fourier beads  

E-print Network

biologically rele- vant reactions, namely, L- to D-alanine amino acid inversion and alcohol acylation by amidesTHE JOURNAL OF CHEMICAL PHYSICS 139, 165104 (2013) Exploring chemical reaction mechanisms through Biotechnology High Performance Computing Software Applications Institute, Telemedicine and Advanced Technology

2013-01-01

143

Results of the 2010 Survey on Teaching Chemical Reaction Engineering  

ERIC Educational Resources Information Center

A survey of faculty teaching the chemical reaction engineering course or sequence during the 2009-2010 academic year at chemical engineering programs in the United States and Canada reveals change in terms of content, timing, and approaches to teaching. The report consists of two parts: first, a statistical and demographic characterization of the…

Silverstein, David L.; Vigeant, Margot A. S.

2012-01-01

144

LIGAND: Database of Chemical Compounds and Reactions in Biological Pathways  

NSDL National Science Digital Library

The Institute for Chemical Research at Kyoto University provides this frequently updated and well-documented database of enzyme reactions. With more than 9,300 entries, the LIGAND Chemical Database includes over 3,700 entries for enzymes (the Enzyme Reaction Database) and 5,600 entries for compounds (Chemical Compound Database). The database is searchable by keyword using DBGET (which supports numerous other databases and gene catalogs as well) and is accompanied by clear instructions. The LIGAND database, updated weekly, may be downloaded via anonymous FTP.

145

The 1st Law of Thermodynamics in Chemical Reactions  

E-print Network

In the previous papers of the author it has been shown that the 1st law of thermodynamics in chemical reactions is the following one: dU=dQ+PdV+SUM In the present paper this theory was developed and it has been shown that the 1st law of thermodynamics in chemical reactions has the following form: dC=-dU+dA and -dU=dQ where dC is the change in the chemical energy, dU is the change in the internal energy. Internal energy is the energy of thermal motion of molecules.

I. A. Stepanov

2000-11-10

146

Atomic structure of clusters through chemical reactions  

SciTech Connect

Techniques for the probing of isolated metal cluster structure through adsorbate binding patterns will be described. The saturation of clusters with reagents such as ammonia and nitrogen provides information on the number of preferred binding sites for these reagents. The dependence of this number on cluster size can suggest particular structural themes. The equilibrium reaction with water can be used to identify cluster sizes having especially enhanced binding for the water molecule. Again, the sequence of cluster sizes showing such enhancement can point to specific cluster structure. The reaction with oxygen can identify cluster sizes having particularly high ionization potentials, and these can be compared to simple models for the electronic structure of metal clusters. Representative applications of these probes to iron, cobalt, nickel, and copper clusters will be discussed. 5 figs.

Riley, S.J.

1991-01-01

147

Dynamics of chemical reactions at cold and ultracold temperatures  

NASA Astrophysics Data System (ADS)

The main characteristics of cold and ultracold chemical reactions are reviewed through the illustrative study of the O(3P) + H2 reaction dynamics. Using separate analytic representations of the lowest H2O(3A'') electronic state which differ essentially by their descriptions of long-range forces, quantum-mechanical scattering calculations show the crucial role played by the van der Waals interaction potential in chemical reactions at low temperatures. Furthermore, the presence of zero-energy resonances is found to significantly enhance chemical reactivity in the ultracold regime. At translational energies comparable to the well depth of the van der Waals potential, initial-state-selected probabilities and excitation functions are characterized by Feshbach resonances arising from the decay of quasibound states supported by the van der Waals well in the entrance channel of the reaction.

Weck, P. F.; Balakrishnan, N.

2006-10-01

148

[Research on chemical reactions during ginseng processing].  

PubMed

As a kind of commonly used traditional Chinese medicine, ginseng has a high reputation at home and abroad. The research of ginseng has been expanded to medicine, pharmacy, biology, food science and other fields, with great achievements in recent years. Ginseng contains ginsenosides, volatile oil, carbohydrates, amino acids, polypeptides, inorganic elements and othser chemical constituents. Each component has extensive physiological activity, and is the base of ginseng's effect. After processing, the complicated changes are taken place in the constituents of ginseng, and some new substances produced. This paper aims to review the studies on chemical constituents and their mechanisms during ginseng processing, and the ideas, methods and the direction of the development of traditional Chinese medicine processing in the future. PMID:25612424

Zhang, Miao; Qin, Kun-Ming; Li, Wei-Dong; Yin, Fang-Zhou; Cai, Hao; Cai, Bao-Chang

2014-10-01

149

Isotopic Effects in Chemical Reactions of Single Ions  

NASA Astrophysics Data System (ADS)

Chemical reactions using laser cooled ions are dominated by quantum effects due to their localized nature. Studying isotopic effects allows for the probing of reaction mechanisms and the topography of potential energy surfaces. Previously, single ion experiments involving Mg+ and HD were done utilizing a nondestructive identification method based on the motional modes of the ions. Our work focuses on reactions with single atomic calcium ions. A novel method that observes the sideband spectra of the 2S1/2 to 2D5/2 transition in Ca+ for reaction detection is discussed. P. F. Staanum et al., Phys. Rev. Lett. 100, 243003 (2008)

Goeders, James E.; Clark, Craig R.; Brown, Kenneth R.

2011-06-01

150

Understanding chemical reactions within a generalized Hamilton-Jacobi framework  

E-print Network

Reaction paths and classical and quantum trajectories are studied within a generalized Hamilton-Jacobi framework, which allows to put on equal footing topology and dynamics in chemical reactivity problems. In doing so, we show how high-dimensional problems could be dealt with by means of Caratheodory plots or how trajectory-based quantum-classical analyses reveal unexpected discrepancies. As a working model, we consider the reaction dynamics associated with a Mueller-Brown potential energy surface, where we focus on the relationship between reaction paths and trajectories as well as on reaction probability calculations from classical and quantum trajectories.

A. S. Sanz; X. Gimenez; J. M. Bofill; S. Miret-Artes

2009-08-13

151

METHODOLOGICAL NOTES: Brusselator — an abstract chemical reaction?  

NASA Astrophysics Data System (ADS)

In this paper we consider the Brusselator and the Sel'kov model, which describes the irreversible reaction of glycolysis in the regime of self-sustained oscillations. We show that these two differently constructed models can be reduced to a single equation — a generalized Rayleigh equation. The physical basis for this generality is investigated. The advantages of this equation as a tool for qualitative and quantitative analyses, as well as the similarities and differences of the solutions realized for each of the two concrete models in the cases of almost harmonic and relaxation self-sustained oscillations, are discussed.

Lavrova, Anastasiya I.; Postnikov, E. B.; Romanovsky, Yurii M.

2009-12-01

152

Matrix isolation as a tool for studying interstellar chemical reactions  

NASA Technical Reports Server (NTRS)

Since the identification of the OH radical as an interstellar species, over 50 molecular species were identified as interstellar denizens. While identification of new species appears straightforward, an explanation for their mechanisms of formation is not. Most astronomers concede that large bodies like interstellar dust grains are necessary for adsorption of molecules and their energies of reactions, but many of the mechanistic steps are unknown and speculative. It is proposed that data from matrix isolation experiments involving the reactions of refractory materials (especially C, Si, and Fe atoms and clusters) with small molecules (mainly H2, H2O, CO, CO2) are particularly applicable to explaining mechanistic details of likely interstellar chemical reactions. In many cases, matrix isolation techniques are the sole method of studying such reactions; also in many cases, complexations and bond rearrangements yield molecules never before observed. The study of these reactions thus provides a logical basis for the mechanisms of interstellar reactions. A list of reactions is presented that would simulate interstellar chemical reactions. These reactions were studied using FTIR-matrix isolation techniques.

Ball, David W.; Ortman, Bryan J.; Hauge, Robert H.; Margrave, John L.

1989-01-01

153

An Efficient Chemical Reaction Optimization Algorithm for Multiobjective Optimization.  

PubMed

Recently, a new metaheuristic called chemical reaction optimization was proposed. This search algorithm, inspired by chemical reactions launched during collisions, inherits several features from other metaheuristics such as simulated annealing and particle swarm optimization. This fact has made it, nowadays, one of the most powerful search algorithms in solving mono-objective optimization problems. In this paper, we propose a multiobjective variant of chemical reaction optimization, called nondominated sorting chemical reaction optimization, in an attempt to exploit chemical reaction optimization features in tackling problems involving multiple conflicting criteria. Since our approach is based on nondominated sorting, one of the main contributions of this paper is the proposal of a new quasi-linear average time complexity quick nondominated sorting algorithm; therebymaking our multiobjective algorithm efficient from a computational cost viewpoint. The experimental comparisons against several other multiobjective algorithms on a variety of benchmark problems involving various difficulties show the effectiveness and the efficiency of this multiobjective version in providing a wellconverged and well-diversified approximation of the Pareto front. PMID:25373137

Bechikh, Slim; Chaabani, Abir; Said, Lamjed Ben

2014-10-30

154

STM CONTROL OF CHEMICAL REACTIONS: Single-Molecule Synthesis  

NASA Astrophysics Data System (ADS)

The fascinating advances in single atom/molecule manipulation with a scanning tunneling microscope (STM) tip allow scientists to fabricate atomic-scale structures or to probe chemical and physical properties of matters at an atomic level. Owing to these advances, it has become possible for the basic chemical reaction steps, such as dissociation, diffusion, adsorption, readsorption, and bond-formation processes, to be performed by using the STM tip. Complete sequences of chemical reactions are able to induce at a single-molecule level. New molecules can be constructed from the basic molecular building blocks on a one-molecule-at-a-time basis by using a variety of STM manipulation schemes in a systematic step-by-step manner. These achievements open up entirely new opportunities in nanochemistry and nanochemical technology. In this review, various STM manipulation techniques useful in the single-molecule reaction process are reviewed, and their impact on the future of nanoscience and technology are discussed.

Hla, Saw-Wai; Rieder, Karl-Heinz

2003-10-01

155

Maximum Probability Reaction Sequences in Stochastic Chemical Kinetic Systems  

PubMed Central

The detailed behavior of many molecular processes in the cell, such as protein folding, protein complex assembly, and gene regulation, transcription and translation, can often be accurately captured by stochastic chemical kinetic models. We investigate a novel computational problem involving these models – that of finding the most-probable sequence of reactions that connects two or more states of the system observed at different times. We describe an efficient method for computing the probability of a given reaction sequence, but argue that computing most-probable reaction sequences is EXPSPACE-hard. We develop exact (exhaustive) and approximate algorithms for finding most-probable reaction sequences. We evaluate these methods on test problems relating to a recently-proposed stochastic model of folding of the Trp-cage peptide. Our results provide new computational tools for analyzing stochastic chemical models, and demonstrate their utility in illuminating the behavior of real-world systems. PMID:21629860

Salehi, Maryam; Perkins, Theodore J.

2010-01-01

156

Maximum Probability Reaction Sequences in Stochastic Chemical Kinetic Systems  

PubMed Central

The detailed behavior of many molecular processes in the cell, such as protein folding, protein complex assembly, and gene regulation, transcription and translation, can often be accurately captured by stochastic chemical kinetic models. We investigate a novel computational problem involving these models – that of finding the most-probable sequence of reactions that connects two or more states of the system observed at different times. We describe an efficient method for computing the probability of a given reaction sequence, but argue that computing most-probable reaction sequences is EXPSPACE-hard. We develop exact (exhaustive) and approximate algorithms for finding most-probable reaction sequences. We evaluate these methods on test problems relating to a recently-proposed stochastic model of folding of the Trp-cage peptide. Our results provide new computational tools for analyzing stochastic chemical models, and demonstrate their utility in illuminating the behavior of real-world systems. PMID:21441987

Salehi, Maryam; Perkins, Theodore J.

2010-01-01

157

Method and apparatus for controlling gas evolution from chemical reactions  

DOEpatents

The present invention is directed toward monitoring a thermally driven gas evolving chemical reaction with an acoustic apparatus. Signals from the acoustic apparatus are used to control a heater to prevent a run-away condition. A digestion module in combination with a robotic arm further automate physical handling of sample material reaction vessels. The invention is especially useful for carrying out sample procedures defined in EPA Methods SW-846. 8 figs.

Skorpik, J.R.; Dodson, M.G.

1999-05-25

158

Method and apparatus for controlling gas evolution from chemical reactions  

DOEpatents

The present invention is directed toward monitoring a thermally driven gas evolving chemical reaction with an acoustic apparatus. Signals from the acoustic apparatus are used to control a heater to prevent a run-away condition. A digestion module in combination with a robotic arm further automate physical handling of sample material reaction vessels. The invention is especially useful for carrying out sample procedures defined in EPA Methods SW-846.

Skorpik, James R. (Kennewick, WA); Dodson, Michael G. (Richland, WA)

1999-01-01

159

Simulation of chemical reaction dynamics on an NMR quantum computer  

E-print Network

Quantum simulation can beat current classical computers with minimally a few tens of qubits and will likely become the first practical use of a quantum computer. One promising application of quantum simulation is to attack challenging quantum chemistry problems. Here we report an experimental demonstration that a small nuclear-magnetic-resonance (NMR) quantum computer is already able to simulate the dynamics of a prototype chemical reaction. The experimental results agree well with classical simulations. We conclude that the quantum simulation of chemical reaction dynamics not computable on current classical computers is feasible in the near future.

Dawei Lu; Nanyang Xu; Ruixue Xu; Hongwei Chen; Jiangbin Gong; Xinhua Peng; Jiangfeng Du

2011-05-21

160

Freezing of Spinodal Decompostion by Irreversible Chemical Growth Reaction  

E-print Network

We present a description of the freezing of spinodal decomposition in systems, which contain simultaneous irreversible chemical reactions, in the hydrodynamic limit approximation. From own results we conclude, that the chemical reaction leads to an onset of spinodal decomposition also in the case of an initial system which is completely miscible and can lead to an extreme retardation of the dynamics of the spinodal decomposition, with the probability of a general freezing of this process, which can be experimetally observed in simultaneous IPN formation.

Michael Schulz; Benjamin Paul

1998-08-10

161

5.0 Application of Chemical Reaction Codes 5.1. Background  

E-print Network

5.1 5.0 Application of Chemical Reaction Codes 5.1. Background Determination of species analyses of water compositions and a competent chemical reaction model. Computerized chemical reaction that may leach from waste, an understanding of the capabilities and application of chemical reaction models

162

The Chemical Reaction Model Recent Developments and Prospects  

Microsoft Academic Search

In 2001, we gave a survey of more than fifteen years of research on the chemical paradigm which had been a source of inspiration\\u000a in many different research areas. The present article presents a digest of recent advances concerning the chemical reaction\\u000a model. We focus to a large extent on: (1) upgrading the basic model to a higher order formalism

Jean-pierre Banâtre; Pascal Fradet; Yann Radenac

2008-01-01

163

CHARACTERIZATION OF CHEMICALLY MODIFIED HYPERTHERMOPHILIC ENZYMES FOR CHEMICAL SYNTHESES AND BIOREMEDIATION REACTIONS  

EPA Science Inventory

Research developments in the area of biocatalysis in organic solvents are expected to greatly expand the role of bioprocessing in chemical synthesis, fuel processing, and bioremediation technologies. Many biological transformation reactions of interest to DOE site remediation inv...

164

Asymmetric chemical reactions by polarized quantum beams  

NASA Astrophysics Data System (ADS)

One of the most attractive hypothesis for the origin of homochirality in terrestrial bio-organic compounds (L-amino acid and D-sugar dominant) is nominated as "Cosmic Scenario"; a chiral impulse from asymmetric excitation sources in space triggered asymmetric reactions on the surfaces of such space materials as meteorites or interstellar dusts prior to the existence of terrestrial life. 1) Effective asymmetric excitation sources in space are proposed as polarized quantum beams, such as circularly polarized light and spin polarized electrons. Circularly polarized light is emitted as synchrotron radiation from tightly captured electrons by intense magnetic field around neutron stars. In this case, either left-or right-handed polarized light can be observed depending on the direction of observation. On the other hand, spin polarized electrons is emitted as beta-ray in beta decay from radioactive nuclei or neutron fireballs in supernova explosion. 2) The spin of beta-ray electrons is longitudinally polarized due to parity non-conservation in the weak interaction. The helicity (the the projection of the spin onto the direction of kinetic momentum) of beta-ray electrons is universally negative (left-handed). For the purpose of verifying the asymmetric structure emergence in bio-organic compounds by polarized quantum beams, we are now carrying out laboratory simulations using circularly polarized light from synchrotron radiation facility or spin polarized electron beam from beta-ray radiation source. 3,4) The target samples are solid film or aqueous solution of racemic amino acids. 1) K.Kobayashi, K.Kaneko, J.Takahashi, Y.Takano, in Astrobiology: from simple molecules to primitive life; Ed. V.Basiuk; American Scientific Publisher: Valencia, 2008. 2) G.A.Gusev, T.Saito, V.A.Tsarev, A.V.Uryson, Origins Life Evol. Biosphere. 37, 259 (2007). 3) J.Takahashi, H.Shinojima, M.Seyama, Y.Ueno, T.Kaneko, K.Kobayashi, H.Mita, M.Adachi, M.Hosaka, M.Katoh, Int. J. Mol. Sci. 10, 3044 (2009). 4) V.I.Burkov, L.A.Goncharova, G.A.Gusev, H.Hashimoto, F.Kaneko, T.Kaneko, K. Kobayashi, H.Mita, E.V.Moiseenko, T.Ogawa, N.G.Poluhina, T.Saito, S.Shima, J.Takahashi, M.Tanaka, Y.Tao, V.A.Tsarev, J.Xu, H.Yabuta, K.Yagi-Watanabe, H.Yan, G.Zhang, Origins Life Evol. Biosphere, 39 295 (2009).

Takahashi, Jun-Ichi; Kobayashi, Kensei

165

Aristotle’s Theory of Chemical Reaction and Chemical Substances  

E-print Network

Abstract. The dominance of atomic structure in modern chemistry has led to an emphasis of the atomic tradition which overshadows what the subject owes to the other traditions. But there are definite features of modern chemical theory which hark back to one or other of the anti-atomistic conceptions. Aristotle maintained that the appearance of genuinely new substances, with properties radically different from the original constituents’, could not be explained by the atomic theory. Some points of his critique of the atomists ’ theory of mixing could still be applied to 19th century atomism, and call for some care in discerning what general ideas have actually been retained from the atomic tradition. He argued that compounds are genuinely homogeneous—i.e. every part of any quantity of a compound is of the same kind as the whole—which led him to maintain that the constituents are not actually, but merely potentially, present in a compound. The theory is not without difficulties of its own. The present paper reviews the way Aristotle motivated his own approach and pursues the idea of constituents being potentially present in a compound. 1

Paul Needham

166

Laboratory studies of O\\/++\\/ reactions of ionospheric importance  

Microsoft Academic Search

The rate constants for the reactions of O(++) ions with several molecules and atoms have been measured. For reaction with N2 the rate constant was found to be (1.6 + or - 0.6) x 10 to the -9th cu cm\\/sec, which agrees very well with an earlier laboratory study but which is considerably larger than the upper limit deduced from

F. Howorka; A. A. Viggiano; E. E. Ferguson; D. L. Albritton; F. C. Fehsenfeld

1979-01-01

167

Chemical pathways in ultracold reactions of SrF molecules  

SciTech Connect

We present a theoretical investigation of the chemical reaction SrF + SrF {yields} products, focusing on reactions at ultralow temperatures. We find that bond swapping SrF + SrF {yields} Sr{sub 2} + F{sub 2} is energetically forbidden at these temperatures. Rather, the only energetically allowed reaction is SrF + SrF {yields} SrF{sub 2} + Sr, and even then only singlet states of the SrF{sub 2} trimer can form. A calculation along a reduced reaction path demonstrates that this abstraction reaction is barrierless and proceeds by one SrF molecule ''handing off'' a fluorine atom to the other molecule.

Meyer, Edmund R.; Bohn, John L. [JILA, NIST, and Department of Physics, University of Colorado, Boulder, Colorado 80309-0440 (United States)

2011-03-15

168

Amplitude Equations and Chemical Reaction-Diffusion Systems  

E-print Network

The paper discusses the use of amplitude equations to describe the spatio-temporal dynamics of a chemical reaction-diffusion system based on an Oregonator model of the Belousov-Zhabotinsky reaction. Sufficiently close to a supercritical Hopf bifurcation the reaction-diffusion equation can be approximated by a complex Ginzburg-Landau equation with parameters determined by the original equation at the point of operation considered. We illustrate the validity of this reduction by comparing numerical spiral wave solutions to the Oregonator reaction-diffusion equation with the corresponding solutions to the complex Ginzburg-Landau equation at finite distances from the bifurcation point. We also compare the solutions at a bifurcation point where the systems develop spatio-temporal chaos. We show that the complex Ginzburg-Landau equation represents the dynamical behavior of the reaction-diffusion equation remarkably well sufficiently far from the bifurcation point for experimental applications to be feasible.

M. Ipsen; F. Hynne; P. G. Soerensen

1997-11-03

169

Favorite Demonstration: Demonstrating Indigo Carmine Oxidation-Reduction Reactions--A Choreography for Chemical Reactions  

NSDL National Science Digital Library

The indigo carmine demonstration (Ferguson et al. 1973), also referred to as a traffic-light demonstration (Flinn Scientific 2007a), is an example of a set of oxidation-reduction reactions that occurs within one solution. This type of demonstration can be used to introduce the concept of chemical reaction to undergraduate nonscience majors. Through their observations guided by the instructor, students begin to develop and construct the following concepts: color changes, reaction rates, reversible reactions, energy requirements (endothermic/exothermic), and equilibrium.

David M. Majerich

2008-03-01

170

Reaction Hamiltonian and state-to-state description of chemical reactions  

SciTech Connect

A chemical reaction is treated as a quantum transition from reactants to products. A specific reaction Hamiltonian (in second quantization formalism) is introduced. The approach leads to Franck-Condon-like factor, and adiabatic method in the framework of the nuclear motion problems. The influence of reagent vibrational state on the product energy distribution has been studied following the reaction Hamiltonian method. Two different cases (fixed available energy and fixed translational energy) are distinguished. Results for several biomolecular reactions are presented. 40 refs., 5 figs.

Ruf, B.A.; Kresin, V.Z.; Lester, W.A. Jr.

1985-08-01

171

Reduction of chemical reaction networks through delay distributions  

NASA Astrophysics Data System (ADS)

Accurate modelling and simulation of dynamic cellular events require two main ingredients: an adequate description of key chemical reactions and simulation of such chemical events in reasonable time spans. Quite logically, posing the right model is a crucial step for any endeavour in Computational Biology. However, more often than not, it is the associated computational costs which actually limit our capabilities of representing complex cellular behaviour. In this paper, we propose a methodology aimed at representing chains of chemical reactions by much simpler, reduced models. The abridgement is achieved by generation of model-specific delay distribution functions, consecutively fed to a delay stochastic simulation algorithm. We show how such delay distributions can be analytically described whenever the system is solely composed of consecutive first-order reactions, with or without additional "backward" bypass reactions, yielding an exact reduction. For models including other types of monomolecular reactions (constitutive synthesis, degradation, or "forward" bypass reactions), we discuss why one must adopt a numerical approach for its accurate stochastic representation, and propose two alternatives for this. In these cases, the accuracy depends on the respective numerical sample size. Our model reduction methodology yields significantly lower computational costs while retaining accuracy. Quite naturally, computational costs increase alongside network size and separation of time scales. Thus, we expect our model reduction methodologies to significantly decrease computational costs in these instances. We anticipate the use of delays in model reduction will greatly alleviate some of the current restrictions in simulating large sets of chemical reactions, largely applicable in pharmaceutical and biological research.

Barrio, Manuel; Leier, André; Marquez-Lago, Tatiana T.

2013-03-01

172

A Review of the Thermodynamic, Transport, and Chemical Reaction Rate Properties of High-temperature Air  

NASA Technical Reports Server (NTRS)

Thermodynamic and transport properties of high temperature air, and the reaction rates for the important chemical processes which occur in air, are reviewed. Semiempirical, analytic expressions are presented for thermodynamic and transport properties of air. Examples are given illustrating the use of these properties to evaluate (1) equilibrium conditions following shock waves, (2) stagnation region heat flux to a blunt high-speed body, and (3) some chemical relaxation lengths in stagnation region flow.

Hansen, C Frederick; Heims, Steve P

1958-01-01

173

Program Helps To Determine Chemical-Reaction Mechanisms  

NASA Technical Reports Server (NTRS)

General Chemical Kinetics and Sensitivity Analysis (LSENS) computer code developed for use in solving complex, homogeneous, gas-phase, chemical-kinetics problems. Provides for efficient and accurate chemical-kinetics computations and provides for sensitivity analysis for variety of problems, including problems involving honisothermal conditions. Incorporates mathematical models for static system, steady one-dimensional inviscid flow, reaction behind incident shock wave (with boundary-layer correction), and perfectly stirred reactor. Computations of equilibrium properties performed for following assigned states: enthalpy and pressure, temperature and pressure, internal energy and volume, and temperature and volume. Written in FORTRAN 77 with exception of NAMELIST extensions used for input.

Bittker, D. A.; Radhakrishnan, K.

1995-01-01

174

Detailed chemical kinetic reaction mechanism for biodiesel components methyl stearate and methyl oleate  

E-print Network

Detailed chemical kinetic reaction mechanism for biodiesel components methyl stearate and methyl are developed for two of the five major components of biodiesel fuel, methyl stearate and methyl oleate renewable sources, can reduce net emissions of greenhouse gases. An important class of biodiesel fuels

Paris-Sud XI, Université de

175

Impact of supersonic and subsonic aircraft on ozone: Including heterogeneous chemical reaction mechanisms  

NASA Technical Reports Server (NTRS)

Preliminary calculations suggest that heterogeneous reactions are important in calculating the impact on ozone from emissions of trace gases from aircraft fleets. In this study, three heterogeneous chemical processes that occur on background sulfuric acid aerosols are included and their effects on O3, NO(x), Cl(x), HCl, N2O5, ClONO2 are calculated.

Kinnison, Douglas E.; Wuebbles, Donald J.

1994-01-01

176

2011 Chemical Reactions at Surfaces Gordon Research Conference  

SciTech Connect

The Gordon Research Conference on Chemical Reactions at Surfaces is dedicated to promoting and advancing the fundamental science of interfacial chemistry and physics by providing surface scientists with the foremost venue for presentation and discussion of research occurring at the frontiers of their fields.

Peter Stair

2011-02-11

177

Quantum and semiclassical theories of chemical reaction rates  

SciTech Connect

A rigorous quantum mechanical theory (and a semiclassical approximation thereto) is described for calculating chemical reaction rates ``directly``, i.e., without having to solve the complete state-to-state reactive scattering problem. The approach has many vestiges of transition state theory, for which it may be thought of as the rigorous generalization.

Miller, W.H. [Univ. of California, Berkeley, CA (United States). Dept. of Chemistry]|[Lawrence Berkeley National Lab., CA (United States). Chemical Sciences Div.

1995-09-01

178

Molecular Codes in Biological and Chemical Reaction Networks  

PubMed Central

Shannon’s theory of communication has been very successfully applied for the analysis of biological information. However, the theory neglects semantic and pragmatic aspects and thus cannot directly be applied to distinguish between (bio-) chemical systems able to process “meaningful” information from those that do not. Here, we present a formal method to assess a system’s semantic capacity by analyzing a reaction network’s capability to implement molecular codes. We analyzed models of chemical systems (martian atmosphere chemistry and various combustion chemistries), biochemical systems (gene expression, gene translation, and phosphorylation signaling cascades), an artificial chemistry, and random reaction networks. Our study suggests that different chemical systems posses different semantic capacities. No semantic capacity was found in the model of the martian atmosphere chemistry, the studied combustion chemistries, and highly connected random networks, i.e. with these chemistries molecular codes cannot be implemented. High semantic capacity was found in the studied biochemical systems and in random reaction networks where the number of second order reactions is twice the number of species. We conclude that our approach can be applied to evaluate the information processing capabilities of a chemical system and may thus be a useful tool to understand the origin and evolution of meaningful information, e.g. in the context of the origin of life. PMID:23372756

Görlich, Dennis; Dittrich, Peter

2013-01-01

179

Interaction between chemical reactions and mixing on various scales  

Microsoft Academic Search

The way in which reagents are mixed can have a large influence on the product distribution of a chemical reaction. This has been analysed earlier when micromixing is the limiting mixing step. Additional segregation at a larger scale has only been treated in detail when the local turbulent dispersion of a feed stream was relevant. Here additional segregation due to

S. J. Hearn

1997-01-01

180

Chemical reaction models for non-equilibrium phase transitions  

Microsoft Academic Search

Chemical model reactions are discussed the steady states of which show the phenomenon of non equilibrium phase transitions. One example shows a phase transition of second order, another one shows a phase transition of first order. If diffusion occurs in the case of first order transition, coexistence of two phases in different domains is possible. For plane boundary layers between

F. Schlögl

1972-01-01

181

WATER AS A REACTION MEDIUM FOR CLEAN CHEMICAL PROCESSES.  

EPA Science Inventory

Green chemistry is a rapid developing new field that provides us a pro-active avenue for the sustainable development of future science and technologies. When designed properly, clean chemical technology can be developed in water as a reaction media. The technologies generated f...

182

Dissipation Scale Fluctuations and Chemical Reaction Rates in Turbulent Flows  

E-print Network

Small separation between reactants, not exceeding $10^{-8}-10^{-7}cm$, is the necessary condition for various chemical reactions. It is shown that random advection and stretching by turbulence leads to formation of scalar-enriched sheets of {\\it strongly fluctuating thickness} $\\eta_{c}$. The molecular-level mixing is achieved by diffusion across these sheets (interfaces) separating the reactants. Since diffusion time scale is $\\tau_{d}\\propto \\eta_{c}^{2}$, the knowledge of probability density $Q(\\eta_{c},Re)$ is crucial for evaluation of chemical reaction rates. In this paper we derive the probability density $Q(\\eta_{c},Re,Sc)$ and predict a transition in the reaction rate behavior from ${\\cal R}\\propto \\sqrt{Re}$ ($Re\\leq 10^{4}$) to the high-Re asymptotics ${\\cal R}\\propto Re^{0}$. The theory leads to an approximate universality of transitional Reynolds number $Re_{tr}\\approx 10^{4}$. It is also shown that if chemical reaction involves short-lived reactants, very strong anomalous fluctuations of the length-scale $\\eta_{c}$ may lead to non-negligibly small reaction rates.

Victor Yakhot

2007-06-29

183

Development of a chemical oxygen - iodine laser with production of atomic iodine in a chemical reaction  

SciTech Connect

The alternative method of atomic iodine generation for a chemical oxygen - iodine laser (COIL) in chemical reactions with gaseous reactants is investigated experimentally. The influence of the configuration of iodine atom injection into the laser cavity on the efficiency of the atomic iodine generation and small-signal gain is studied. (lasers)

Censky, M; Spalek, O; Jirasek, V; Kodymova, J [Institute of Physics, Czech Academy of Sciences, Prague (Czech Republic); Jakubec, I [Institute of Inorganic Chemistry, Czech Academy of Sciences, Rez (Czech Republic)

2009-11-30

184

M. Bahrami ENSC 461 (S 11) Chemical Reactions 1 Chemical Reactions  

E-print Network

mass. Theoretical (Stoichiometric) Air The minimum amount of air needed for the complete combustion the reaction. Products: are the components that exist after the reaction. Ignition temperature: the minimum of a fuel is called the stoichiometric or theoretical air. A combustion process is complete if all

Bahrami, Majid

185

Moment equations for chemical reactions on interstellar dust grains  

E-print Network

While most chemical reactions in the interstellar medium take place in the gas phase, those occurring on the surfaces of dust grains play an essential role. Chemical models based on rate equations including both gas phase and grain surface reactions have been used in order to simulate the formation of chemical complexity in interstellar clouds. For reactions in the gas phase and on large grains, rate equations, which are highly efficient to simulate, are an ideal tool. However, for small grains under low flux, the typical number of atoms or molecules of certain reactive species on a grain may go down to order one or less. In this case the discrete nature of the opulations of reactive species as well as the fluctuations become dominant, thus the mean-field approximation on which the rate equations are based does not apply. Recently, a master equation approach, that provides a good description of chemical reactions on interstellar dust grains, was proposed. Here we present a related approach based on moment equations that can be obtained from the master equation. These equations describe the time evolution of the moments of the distribution of the population of the various chemical species on the grain. An advantage of this approach is the fact that the production rates of molecular species are expressed directly in terms of these moments. Here we use the moment equations to calculate the rate of molecular hydrogen formation on small grains. It is shown that the moment equation approach is efficient in this case in which only a single reactive specie is involved. The set of equations for the case of two species is presented and the difficulties in implementing this approach for complex reaction networks involving multiple species are discussed.

Azi Lipshtat; Ofer Biham

2002-12-09

186

Understanding the kinetics of spin-forbidden chemical reactions.  

PubMed

Many chemical reactions involve a change in spin-state and are formally forbidden. This article summarises a number of previously published applications showing that a form of Transition State Theory (TST) can account for the kinetics of these reactions. New calculations for the emblematic spin-forbidden reaction HC + N(2) are also reported. The observed reactivity is determined by two factors. The first is the critical energy required for reaction to occur, which in spin-forbidden reactions is often defined by the relative energy of the Minimum Energy Crossing Point (MECP) between potential energy surfaces corresponding to the different spin states. The second factor is the probability of hopping from one surface to the other in the vicinity of the crossing region, which is largely defined by the spin-orbit coupling matrix element between the two electronic wavefunctions. The spin-forbidden transition state theory takes both factors into account and gives good results. The shortcomings of the theory, which are largely analogous to those of standard TST, are discussed. Finally, it is shown that in cases where the surface-hopping probability is low, the kinetics of spin-forbidden reactions will be characterised by unusually unfavourable entropies of activation. As a consequence, reactions involving a spin-state change can be expected to compete poorly with spin-allowed reactions at high temperatures (or energies). PMID:17199148

Harvey, Jeremy N

2007-01-21

187

Parameter estimation in complex flows with chemical reactions  

Microsoft Academic Search

The estimation of unknown parameters in engineering and scientific models continues to be of great importance in order to validate them to available experimental data. These parameters of concern cannot be known beforehand, but must be measured experimentally, variables such as chemical species concentrations, pressures, or temperatures as examples. Particularly, in chemically reacting flows, the estimation of kinetic rate parameters

Daniel J. Robinson

2010-01-01

188

INVESTIGATION OF IMPORTANT HYDROXYL RADICAL REACTIONS IN THE PERTURBED TROPOSPHERE  

EPA Science Inventory

The flash-photolysis resonance fluorescence technique was used to study the reaction kinetics of hydroxyl radicals with ten aromatic and six olefinic hydrocarbons at 298 K and several diluent gas pressures. The aromatic compounds that were studied include benzene, toluene, ethylb...

189

On the Influence of Uncertainties in Chemical Reaction Rates on Results of the Astrochemical Modelling  

E-print Network

With the chemical reaction rate database UMIST95 (Millar et al. 1997) we analyze how uncertainties in rate constants of gas-phase chemical reactions influence the modelling of molecular abundances in the interstellar medium. Random variations are introduced into the rate constants to estimate the scatter in theoretical abundances. Calculations are performed for dark and translucent molecular clouds where gas phase chemistry is adequate. Similar approach was used by Pineau des Forets & Roueff (2000) for the study of chemical bistability. All the species are divided into 6 sensitivity groups according to the value of the scatter in their model abundances computed with varied rate constants. It is shown that the distribution of species within these groups depends on the number of atoms in a molecule and on the adopted physical conditions. The simple method is suggested which allows to single out reactions that are most important for the evolution of a given species.

A. I. Vasyunin; A. M. Sobolev; D. S. Wiebe; D. A. Semenov

2003-11-19

190

Modelling of gas-solid reaction—Coupling of heat and mass transfer with chemical reaction  

Microsoft Academic Search

A general gas-solid reaction model is formulated. This work is the further development of the previous modelling work of Mazet (1988, Ph.D. Thesis, University of Perpignan) and Goetz (1991, Ph.D. Thesis, University of Perpignan) to simulate reversible gas-solid reactions that have been extensively applied to the new chemical heat pump technology developed at CNRS-IMP. In the present paper, a general

Hui-Bo Lu; Nathalie Mazet; Bernard Spinner

1996-01-01

191

Chemical reactions studied at ultra-low temperature in liquid helium clusters  

SciTech Connect

Low-temperature reaction rates are important ingredients for astrophysical reaction networks modeling the formation of interstellar matter in molecular clouds. Unfortunately, such data is difficult to obtain by experimental means. In an attempt to study low-temperature reactions of astrophysical interest, we have investigated relevant reactions at ultralow temperature in liquid helium droplets. Being prepared by supersonic expansion of helium gas at high pressure through a nozzle into a vacuum, large helium clusters in the form of liquid droplets constitute nano-sized reaction vessels for the study of chemical reactions at ultra-low temperature. If the normal isotope {sup 4}He is used, the helium droplets are superfluid and characterized by a constant temperature of 0.37 K. Here we present results obtained for Mg, Al, and Si reacting with O{sub 2}. Mass spectrometry was employed to characterize the reaction products. As it may be difficult to distinguish between reactions occurring in the helium droplets before they are ionized and ion-molecule reactions taking place after the ionization, additional techniques were applied to ensure that the reactions actually occurred in the helium droplets. This information was provided by measuring the chemiluminescence light emitted by the products, the evaporation of helium atoms by the release of the reaction heat, or by laser-spectroscopic identification of the reactants and products.

Huisken, Friedrich; Krasnokutski, Serge A. [Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the University of Jena, Institute of Solid State Physics, Helmholtzweg 3, D-07743 Jena (Germany)

2012-11-27

192

"Kinetics of Chemical Reactions in Environmental Systems: Research Needs and Challenges"  

E-print Network

"Kinetics of Chemical Reactions in Environmental Systems: Research Needs and Challenges" Donald(oid)s, nutrients, radionuclides, and organic chemicals have shown that reaction rates are initially rapid followed by a slow approach to a steady state. The rapid reaction has been ascribed to chemical reactions and film

Sparks, Donald L.

193

Theory of rotational transition in atom-diatom chemical reaction  

NASA Astrophysics Data System (ADS)

Rotational transition in atom-diatom chemical reaction is theoretically studied. A new approximate theory (which we call IOS-DW approximation) is proposed on the basis of the physical idea that rotational transition in reaction is induced by the following two different mechanisms: rotationally inelastic half collision in both initial and final arrangement channels, and coordinate transformation in the reaction zone. This theory gives a fairy compact expression for the state-to-state transition probability. Introducing the additional physically reasonable assumption that reaction (particle rearrangement) takes place in a spatially localized region, we have reduced this expression into a simpler analytical form which can explicitly give overall rotational state distribution in reaction. Numerical application was made to the H+H2 reaction and demonstrated its effectiveness for the simplicity. A further simplified most naive approximation, i.e., independent events approximation was also proposed and demonstrated to work well in the test calculation of H+H2. The overall rotational state distribution is expressed simply by a product sum of the transition probabilities for the three consecutive processes in reaction: inelastic transition in the initial half collision, transition due to particle rearrangement, and inelastic transition in the final half collision.

Nakamura, Masato; Nakamura, Hiroki

1989-05-01

194

A self-organized chemical model and reaction cascade.  

PubMed

Some reaction cascades in biological systems are analyzed by a self-organized chemical model, an autocatalytic reaction. This model is described by the coupling of a primary system which stabilizes the initial stage of the reaction rapidly and a partial system which controls the primary system slowly. By the internal force caused by a trigger above the threshold, the coupled system in near-equilibrium is broken and changed into a new state. From the rate equation for the coupled system, a dimensionless nonlinear state equation, n = -n3 - un - v, is derived, where n is the concentration of intermediate, and u, v are dynamic variables of the system. This equation is similar to a nonequilibrium tri-molecular reaction. By using this chemical network theory, fibrin polymerization. F + F----fm----fp + X, where F is a fibrinogen molecule, fm is a fibrin monomer, fp is fibrin polymer, and X is small peptides released from fibrinogen, is discussed as an excellent example of the enzyme reaction cascade. PMID:3419174

Tokimoto, T; Shirane, K

1988-01-01

195

Ris-M-2533 THE INFLUENCE OP CHEMICAL REACTIONS ON THE NOBILITY OF RADIO-  

E-print Network

Risø-M-2533 THE INFLUENCE OP CHEMICAL REACTIONS ON THE NOBILITY OF RADIO- NUCLIDES of the kinetics of chemical and physico -chemical reactions on the mobility of radionuclides in the terrestrial Roskilde, Denmark #12;INIS descriptors; ADSORPTION; C CODES; CHEMICAL REACTION KINETICS; COMPUTERIZED

196

Complex wave patterns in an effective reaction-diffusion model for chemical reactions in microemulsions  

NASA Astrophysics Data System (ADS)

An effective medium theory is employed to derive a simple qualitative model of a pattern forming chemical reaction in a microemulsion. This spatially heterogeneous system is composed of water nanodroplets randomly distributed in oil. While some steps of the reaction are performed only inside the droplets, the transport through the extended medium occurs by diffusion of intermediate chemical reactants as well as by collisions of the droplets. We start to model the system with heterogeneous reaction-diffusion equations and then derive an equivalent effective spatially homogeneous reaction-diffusion model by using earlier results on homogenization in heterogeneous reaction-diffusion systems [S. Alonso, M. Bär, and R. Kapral, J. Chem. Phys. 134, 214102 (2009)]. We study the linear stability of the spatially homogeneous state in the resulting effective model and obtain a phase diagram of pattern formation, that is qualitatively similar to earlier experimental results for the Belousov-Zhabotinsky reaction in an aerosol OT (AOT)-water-in-oil microemulsion [V. K. Vanag and I. R. Epstein, Phys. Rev. Lett. 87, 228301 (2001)]. Moreover, we reproduce many patterns that have been observed in experiments with the Belousov-Zhabotinsky reaction in an AOT oil-in-water microemulsion by direct numerical simulations.

Alonso, Sergio; John, Karin; Bär, Markus

2011-03-01

197

A statistical model for predicting thermal chemical reaction rate  

NASA Astrophysics Data System (ADS)

A simple model based on the statistics of individual atoms [Europhys. Lett. 94 40002 (2011)] or molecules [Chin. Phys. Lett. 29 080504 (2012)] was used to predict chemical reaction rates without empirical parameters, and its physical basis was further investigated both theoretically and via MD simulations. The model was successfully applied to some reactions of extensive experimental data, showing that the model is significantly better than the conventional transition state theory. It is worth noting that the prediction of the model on ab initio level is much easier than the transition state theory or unimolecular RRKM theory.

Lin, Zheng-Zhe; Li, Wang-Yao; Ning, Xi-Jing

2014-05-01

198

Independent research as a teaching tool in graduate chemical reaction engineering. Case study: Modelling isomerization of unsaturated fatty acids with catalyst deactivation  

Microsoft Academic Search

Chemical reaction engineering is one of the most important courses in any chemical engineering program, dealing with many fundamental concepts that can be applied in a variety of fields related to chemical engineering. Given the broad variety of backgrounds of the students in graduate programs, teaching complex reaction engineering applications is challenging. In this paper, we report on our experience

Andrés Mahecha-Botero; Stephen Reaume; John R. Grace; Naoko Ellis

2011-01-01

199

Laser studies of chemical reaction and collision processes  

SciTech Connect

This work has concentrated on several interrelated projects in the area of laser photochemistry and photophysics which impinge on a variety of questions in combustion chemistry and general chemical kinetics. Infrared diode laser probes of the quenching of molecules with {open_quotes}chemically significant{close_quotes} amounts of energy in which the energy transferred to the quencher has, for the first time, been separated into its vibrational, rotational, and translational components. Probes of quantum state distributions and velocity profiles for atomic fragments produced in photodissociation reactions have been explored for iodine chloride.

Flynn, G. [Columbia Univ., New York, NY (United States)

1993-12-01

200

Tuning ultracold chemical reactions via Rydberg-dressed interactions.  

PubMed

We show that ultracold chemical reactions with an activation barrier can be tuned using Rydberg-dressed interactions. Scattering in the ultracold regime is sensitive to long-range interactions, especially when weakly bound (or quasibound) states exist near the collision threshold. We investigate how, by Rydberg dressing a reactant, one enhances its polarizability and modifies the long-range van der Waals collision complex, which can alter chemical reaction rates by shifting the position of near-threshold bound states. We carry out a full quantum mechanical scattering calculation for the benchmark system H(2)+D, and show that resonances can be moved substantially and that rate coefficients at cold and ultracold temperatures can be increased by several orders of magnitude. PMID:25062202

Wang, Jia; Byrd, Jason N; Simbotin, Ion; Côté, R

2014-07-11

201

Tabletop imaging of structural evolutions in chemical reactions  

E-print Network

The introduction of femto-chemistry has made it a primary goal to follow the nuclear and electronic evolution of a molecule in time and space as it undergoes a chemical reaction. Using Coulomb Explosion Imaging we have shot the first high-resolution molecular movie of a to and fro isomerization process in the acetylene cation. So far, this kind of phenomenon could only be observed using VUV light from a Free Electron Laser [Phys. Rev. Lett. 105, 263002 (2010)]. Here we show that 266 nm ultrashort laser pulses are capable of initiating rich dynamics through multiphoton ionization. With our generally applicable tabletop approach that can be used for other small organic molecules, we have investigated two basic chemical reactions simultaneously: proton migration and C=C bond-breaking, triggered by multiphoton ionization. The experimental results are in excellent agreement with the timescales and relaxation pathways predicted by new and definitively quantitative ab initio trajectory simulations.

Ibrahim, Heide; Beaulieu, Samuel; Schmidt, Bruno E; Thiré, Nicolas; Bisson, Éric; Hebeisen, Christoph T; Wanie, Vincent; Giguére, Mathieu; Kieffer, Jean-Claude; Sanderson, Joseph; Schuurman, Michael S; Légaré, François

2014-01-01

202

Tuning Ultracold Chemical Reactions via Rydberg-Dressed Interactions  

NASA Astrophysics Data System (ADS)

We show that ultracold chemical reactions with an activation barrier can be tuned using Rydberg-dressed interactions. Scattering in the ultracold regime is sensitive to long-range interactions, especially when weakly bound (or quasibound) states exist near the collision threshold. We investigate how, by Rydberg dressing a reactant, one enhances its polarizability and modifies the long-range van der Waals collision complex, which can alter chemical reaction rates by shifting the position of near-threshold bound states. We carry out a full quantum mechanical scattering calculation for the benchmark system H2+D, and show that resonances can be moved substantially and that rate coefficients at cold and ultracold temperatures can be increased by several orders of magnitude.

Wang, Jia; Byrd, Jason N.; Simbotin, Ion; Côté, R.

2014-07-01

203

Controlling ultracold chemical reactions via Rydberg-dressed interactions  

E-print Network

We show that ultracold chemical reactions can be manipulated and controlled by using Rydberg-dressed interactions. Scattering in the ultracold regime is sensitive to long-range interactions, especially when weakly bound (or quasi-bound) states exist near the collision threshold. We investigate how, by Rydberg-dressing a reactant, one enhances its polarizability and modifies the long-range van der Waals collision complex, which can alter chemical reaction rates by shifting the position of near threshold bound states. We carry out a full quantum mechanical scattering calculation for the benchmark system H$_2$+D, and show that resonances can be moved substantially and that rate coefficients at cold and ultracold temperatures can be increased by several orders of magnitude.

Jia Wang; Jason N. Byrd; Ion Simbotin; R. Côté

2014-03-24

204

Exploring chemical reaction mechanisms through harmonic Fourier beads path optimization  

NASA Astrophysics Data System (ADS)

Here, we apply the harmonic Fourier beads (HFB) path optimization method to study chemical reactions involving covalent bond breaking and forming on quantum mechanical (QM) and hybrid QM/molecular mechanical (QM/MM) potential energy surfaces. To improve efficiency of the path optimization on such computationally demanding potentials, we combined HFB with conjugate gradient (CG) optimization. The combined CG-HFB method was used to study two biologically relevant reactions, namely, L- to D-alanine amino acid inversion and alcohol acylation by amides. The optimized paths revealed several unexpected reaction steps in the gas phase. For example, on the B3LYP/6-31G(d,p) potential, we found that alanine inversion proceeded via previously unknown intermediates, 2-iminopropane-1,1-diol and 3-amino-3-methyloxiran-2-ol. The CG-HFB method accurately located transition states, aiding in the interpretation of complex reaction mechanisms. Thus, on the B3LYP/6-31G(d,p) potential, the gas phase activation barriers for the inversion and acylation reactions were 50.5 and 39.9 kcal/mol, respectively. These barriers determine the spontaneous loss of amino acid chirality and cleavage of peptide bonds in proteins. We conclude that the combined CG-HFB method further advances QM and QM/MM studies of reaction mechanisms.

Khavrutskii, Ilja V.; Smith, Jason B.; Wallqvist, Anders

2013-10-01

205

Chemical Characterization and Reactivity of Fuel-Oxidizer Reaction Product  

NASA Technical Reports Server (NTRS)

Fuel-oxidizer reaction product (FORP), the product of incomplete reaction of monomethylhydrazine and nitrogen tetroxide propellants prepared under laboratory conditions and from firings of Shuttle Reaction Control System thrusters, has been characterized by chemical and thermal analysis. The composition of FORP is variable but falls within a limited range of compositions that depend on three factors: the fuel-oxidizer ratio at the time of formation; whether the composition of the post-formation atmosphere is reducing or oxidizing; and the reaction or post-reaction temperature. A typical composition contains methylhydrazinium nitrate, ammonium nitrate, methylammonium nitrate, and trace amounts of hydrazinium nitrate and 1,1-dimethylhydrazinium nitrate. Thermal decomposition reactions of the FORP compositions used in this study were unremarkable. Neither the various compositions of FORP, the pure major components of FORP, nor mixtures of FORP with propellant system corrosion products showed any unusual thermal activity when decomposed under laboratory conditions. Off-limit thruster operations were simulated by rapid mixing of liquid monomethylhydrazine and liquid nitrogen tetroxide in a confined space. These tests demonstrated that monomethylhydrazine, methylhydrazinium nitrate, ammonium nitrate, or Inconel corrosion products can induce a mixture of monomethylhydrazine and nitrogen tetroxide to produce component-damaging energies. Damaging events required FORP or metal salts to be present at the initial mixing of monomethylhydrazine and nitrogen tetroxide.

David, Dennis D.; Dee, Louis A.; Beeson, Harold D.

1997-01-01

206

Implementation of a vibrationally linked chemical reaction model for DSMC  

NASA Technical Reports Server (NTRS)

A new procedure closely linking dissociation and exchange reactions in air to the vibrational levels of the diatomic molecules has been implemented in both one- and two-dimensional versions of Direct Simulation Monte Carlo (DSMC) programs. The previous modeling of chemical reactions with DSMC was based on the continuum reaction rates for the various possible reactions. The new method is more closely related to the actual physics of dissociation and is more appropriate to the particle nature of DSMC. Two cases are presented: the relaxation to equilibrium of undissociated air initially at 10,000 K, and the axisymmetric calculation of shuttle forebody heating during reentry at 92.35 km and 7500 m/s. Although reaction rates are not used in determining the dissociations or exchange reactions, the new method produces rates which agree astonishingly well with the published rates derived from experiment. The results for gas properties and surface properties also agree well with the results produced by earlier DSMC models, equilibrium air calculations, and experiment.

Carlson, A. B.; Bird, Graeme A.

1994-01-01

207

Molecular-level simulations of chemical reaction equilibrium and diffusion in slit and cylindrical nanopores: model dimerisation reactions  

E-print Network

Molecular-level simulations of chemical reaction equilibrium and diffusion in slit and cylindrical study of the effects of confinement on chemical reaction equilibrium and diffusion in both slit, and therefore assumed that chemical equilibrium is effectively maintained and unperturbed at all times

Lisal, Martin

208

Chemical dynamics in the gas phase: Time-dependent quantum mechanics of chemical reactions  

SciTech Connect

A major goal of this research is to obtain an understanding of the molecular reaction dynamics of three and four atom chemical reactions using numerically accurate quantum dynamics. This work involves: (i) the development and/or improvement of accurate quantum mechanical methods for the calculation and analysis of the properties of chemical reactions (e.g., rate constants and product distributions), and (ii) the determination of accurate dynamical results for selected chemical systems, which allow one to compare directly with experiment, determine the reliability of the underlying potential energy surfaces, and test the validity of approximate theories. This research emphasizes the use of recently developed time-dependent quantum mechanical methods, i.e. wave packet methods.

Gray, S.K. [Argonne National Laboratory, IL (United States)

1993-12-01

209

Great Chemistry! How is Energy Released in Chemical Reactions?  

NSDL National Science Digital Library

The Materials Science and Technology Teacher's Workshop (MAST) provides this activity to look at the ways in which energy is released in chemical reactions. The class will examine plaster of paris, bromthymol blue and the hydration of baking soda. Students will observe the temperature changes of each.The lesson includes step by step directions for the experiment. Discussion questions and teacher notes are also included.

210

Chemical Reaction-Induced Multi-molecular Polarization (CRIMP)  

PubMed Central

Here we present a novel hyperpolarization method, Chemical Reaction-Induced Multi-molecular Polarization (CRIMP), which could be applied to the study of several in vivo processes simultaneously including glycolysis, TCA cycle, fatty acid synthesis and pH mapping. Through the use of non-enzymatic decarboxylation, we generate four hyperpolarized imaging agents from hyperpolarized 1,2-13C pyruvic acid. PMID:25224323

Lee, Youngbok; Zacharias Millward, Niki M.; Piwnica-Worms, David

2015-01-01

211

Separation of the isotopes of boron by chemical exchange reactions  

DOEpatents

The isotopes of boron, {sup 10}B and {sup 11}B, are separated by means of a gas-liquid chemical exchange reaction involving the isotopic equilibrium between gaseous BF{sub 3} and a liquid BF{sub 3} donor molecular addition complex formed between BF{sub 3} gas and a donor chosen from the group consisting of: nitromethane, acetone, methyl isobutyl ketone, or diisobutyl ketone. 1 Fig.

McCandless, F.P.; Herbst, R.S.

1995-05-30

212

Chemical reaction-induced multi-molecular polarization (CRIMP).  

PubMed

Here we present a novel hyperpolarization method, Chemical Reaction-Induced Multi-molecular Polarization (CRIMP), which could be applied to the study of several in vivo processes simultaneously including glycolysis, TCA cycle, fatty acid synthesis and pH mapping. Through the use of non-enzymatic decarboxylation, we generate four hyperpolarized imaging agents from hyperpolarized 1,2-(13)C pyruvic acid. PMID:25224323

Lee, Y; Zacharias, N M; Piwnica-Worms, D; Bhattacharya, P K

2014-11-01

213

Chemical reactions in viscous liquids under space conditions  

Microsoft Academic Search

A long-term human flight needs a large-size space ships with artificial self-regulating ecological life-support system. The best way for creation of large-size space ship is a synthesis of light construction on Earth orbit, that does not need a high energy transportation carriers from Earth surface. The construction can be created by the way of chemical polymerisation reaction under space environment.

A. Kondyurin; B. Lauke; E. Richter

2004-01-01

214

Stochastic Generator of Chemical Structure. 3. Reaction Network Generation  

SciTech Connect

A new method to generate chemical reaction network is proposed. The particularity of the method is that network generation and mechanism reduction are performed simultaneously using sampling techniques. Our method is tested for hydrocarbon thermal cracking. Results and theoretical arguments demonstrate that our method scales in polynomial time while other deterministic network generator scale in exponential time. This finding offers the possibility to investigate complex reacting systems such as those studied in petroleum refining and combustion.

FAULON,JEAN-LOUP; SAULT,ALLEN G.

2000-07-15

215

Separation of the isotopes of boron by chemical exchange reactions  

DOEpatents

The isotopes of boron, .sup.10 B and .sup.11 B, are separated by means of a gas-liquid chemical exchange reaction involving the isotopic equilibrium between gaseous BF.sub.3 and a liquid BF.sub.3 . donor molecular addition complex formed between BF.sub.3 gas and a donor chosen from the group consisting of: nitromethane, acetone, methyl isobutyl ketone, or diisobutyl ketone.

McCandless, Frank P. (Bozeman, MT); Herbst, Ronald S. (Idaho Falls, ID)

1995-01-01

216

Gamma and the Chemical Reaction Model: Fifteen Years After  

Microsoft Academic Search

Gamma was originally proposed in 1986 as a formalism for the definition of programs without artificial sequentiality. The\\u000a basic idea underlying the formalism is to describe computation as a form of chemical reaction on a collection of individual pieces of data. Due to the very minimal nature of the language, and its absence of sequential\\u000a bias, it has been possible

Jean-Pierre Ban?tre; Pascal Fradet; Daniel Le Métayer

217

The nature of chemical reaction-driven tip-streaming  

NASA Astrophysics Data System (ADS)

The discovery of chemical reaction-driven tip-streaming (also known as "an amazing drop") was made about a decade ago during measurements of the dynamic interfacial tension of a water-alkali pendant droplet immersed in oil-linoleic acid. A plausible explanation for this self-sustained ejection of micron sized droplets from the tip of the macroscopic pendant drop was offered at that time and attributed to Marangoni stresses driving the reaction-produced surfactant along the interface. Later, asymptotic theory based on the analysis of a complete fluid dynamical formulation supported this hypothesis. As this discovery promised a way of microdroplet generation without the need for complex microchannel geometries or externally imposed flow or electric fields, we were recently motivated to study the influence of the reagent concentrations and reaction rate on the droplet generation. However, in an attempt to recreate the original experiments, we revealed that the cause for tip-streaming is not what it originally seemed to be. This led to a series of experiments clarifying the role of the Marangoni stresses and the crucial differences from similar phenomena. As the mechanism by which the phenomenon was originally thought to operate was supported by recent theoretical studies, the present work leads to new intriguing questions of existence and conditions under which a chemical reaction alone can drive Marangoni stresses capable of self-sustaining the process of tip-streaming.

Mayer, H. C.; Krechetnikov, R.

2013-05-01

218

Physio-chemical reactions in recycle aggregate concrete.  

PubMed

Concrete waste constitutes the major proportion of construction waste at about 50% of the total waste generated. An effective way to reduce concrete waste is to reuse it as recycled aggregate (RA) for the production of recycled aggregate concrete (RAC). This paper studies the physio-chemical reactions of cement paste around aggregate for normal aggregate concrete (NAC) and RAC mixed with normal mixing approach (NMA) and two-stage mixing approach (TSMA) by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Four kinds of physio-chemical reactions have been recorded from the concrete samples, including the dehydration of C(3)S(2)H(3), iron-substituted ettringite, dehydroxylation of CH and development of C(6)S(3)H at about 90 degrees C, 135 degrees C, 441 degrees C and 570 degrees C, respectively. From the DSC results, it is confirmed that the concrete samples with RA substitution have generated less amount of strength enhancement chemical products when compared to those without RA substitution. However, the results from the TSMA are found improving the RAC quality. The pre-mix procedure of the TSMA can effectively develop some strength enhancing chemical products including, C(3)S(2)H(3), ettringite, CH and C(6)S(3)H, which shows that RAC made from the TSMA can improve the hydration processes. PMID:18718710

Tam, Vivian W Y; Gao, X F; Tam, C M; Ng, K M

2009-04-30

219

Significance of vapor phase chemical reactions on CVD rates predicted by chemically frozen and local thermochemical equilibrium boundary layer theories  

NASA Technical Reports Server (NTRS)

This paper investigates the role played by vapor-phase chemical reactions on CVD rates by comparing the results of two extreme theories developed to predict CVD mass transport rates in the absence of interfacial kinetic barrier: one based on chemically frozen boundary layer and the other based on local thermochemical equilibrium. Both theories consider laminar convective-diffusion boundary layers at high Reynolds numbers and include thermal (Soret) diffusion and variable property effects. As an example, Na2SO4 deposition was studied. It was found that gas phase reactions have no important role on Na2SO4 deposition rates and on the predictions of the theories. The implications of the predictions of the two theories to other CVD systems are discussed.

Gokoglu, Suleyman A.

1988-01-01

220

Automated microreactor system for reaction development and online optimization of chemical processes  

E-print Network

Developing the optimal conditions for chemical reactions that are common in fine chemical and pharmaceutics is a difficult and expensive task. Because syntheses in these fields have multiple reaction pathways, a significant ...

McMullen, Jonathan Patrick

2010-01-01

221

EYE MOVEMENT AND REACTION TIME ARE BOTH IMPORTANT IN ASSESSMENT OF DIALOG BOX USABILITY  

E-print Network

. Traditional measures and eye movement patterns yielded different results: Reaction time analyses suggestedEYE MOVEMENT AND REACTION TIME ARE BOTH IMPORTANT IN ASSESSMENT OF DIALOG BOX USABILITY Bruce N and reaction time) were combined with eye movement patterns to study button placement and highlighting

222

Importance of toxicokinetics for interspecies variation in sensitivity to chemicals.  

PubMed

Interspecies variation in sensitivity to synthetic chemicals can be orders of magnitude large. Species traits causing the variation can be related to toxicokinetics (uptake, distribution, biotransformation, elimination) or toxicodynamics (interaction with biological target sites). We present an approach to systematically measure and model the contribution of uptake, biotransformation, internal distribution, and elimination kinetics toward species sensitivity differences. The aim is to express sensitivity as target tissue specific, internal lethal concentrations. A case study with the pesticides diazinon, imidacloprid, and propiconazole and the aquatic invertebrates Gammarus pulex, Gammarus fossarum, and Lymnaea stagnalis illustrates the approach. L. stagnalis accumulates more pesticides than Gammaridae when measured in whole organisms but less in target tissues such as the nervous system. Toxicokinetics, i.e. biotransformation and distribution, explain the higher tolerance of L. stagnalis to the insecticide diazinon when compared to Gammaridae. L. stagnalis was again more tolerant to the other neurotoxicant imidacloprid; however, the difference in sensitivity could not be explained by toxicokinetics alone, indicating the importance of toxicodynamic differences. Sensitivity to propiconazole was comparable among all species and, when expressed as internal lethal concentrations, falls in the range of baseline toxicity. PMID:24758734

Nyman, Anna-Maija; Schirmer, Kristin; Ashauer, Roman

2014-05-20

223

[Anaphylactic reactions to low-molecular weight chemicals].  

PubMed

Low-molecular weight chemicals (haptens) include a large group of chemical compounds occurring in work environment, items of everyday use (cleaning products, clothing, footwear, gloves, furniture), jewelry (earrings, bracelets), drugs, especially in cosmetics. They cause type IV hypersensitive reactions. During the induction phase of delayed-type hypersensitivity, haptens form complexes with skin proteins. After internalization through antigen presenting cells, they are bound to MHC class II molecules. Next, they are exposed against specific T-lymphocytes, what triggers activation of Th1 cells mainly. After repeating exposition to that hapten, during effector phase, Th1 induce production of cytokines affecting non-specific inflammatory cells. Usually, it causes contact dermatitis. However, occasionally incidence of immediate generalized reactions after contact with some kinds of haptens is noticed. A question arises, how the hapten does induce symptoms which are typical for anaphylaxis, and what contributes to amplification of this mechanism. It seems that this phenomenon arises from pathomechanism occurring in contact urticaria syndrome in which an anaphylactic reaction may be caused either by contact of sensitized skin with protein antigens, high-molecular weight allergens, or haptens. One of the hypotheses indicates the leading role of basophiles in this process. Their contact with haptens, may cause to release mediators of immediate allergic reaction (histamine, eicosanoids) and to produce cytokines corresponding to Th2 cells profile. Furthermore, Th17 lymphocytes secreting pro-inflammatory interleukin-17 might be engaged into amplifying hypersensitivity into immediate reactions and regulatory T-cells may play role in the process, due to insufficient control of the activity of effector cells. PMID:25661919

Nowak, Daria; Panaszek, Bernard

2015-01-01

224

Holistic Metrics for Assessment of the Greenness of Chemical Reactions in the Context of Chemical Education  

ERIC Educational Resources Information Center

Two new semiquantitative green chemistry metrics, the green circle and the green matrix, have been developed for quick assessment of the greenness of a chemical reaction or process, even without performing the experiment from a protocol if enough detail is provided in it. The evaluation is based on the 12 principles of green chemistry. The…

Ribeiro, M. Gabriela T. C.; Machado, Adelio A. S. C.

2013-01-01

225

Mechanisms for Laser Control of Chemical Reactions Ben R. Torralva and Roland E. Allen  

E-print Network

Introduction For decades, chemists have dreamed of using lasers to selectively control chemical reactionsMechanisms for Laser Control of Chemical Reactions Ben R. Torralva and Roland E. Allen Department the potential for control of chemical reactions, through various mechanisms which include the following: (i

Allen, Roland E.

226

Building robust chemical reaction mechanisms : next generation of automatic model construction software  

E-print Network

Building proper reaction mechanisms is crucial to model the system dynamic properties for many industrial processes with complex chemical reaction phenomena. Because of the complexity of a reaction mechanism, computer-aided ...

Song, Jing, 1972-

2004-01-01

227

Chemical reactions of mercury in combustion flue gases  

Microsoft Academic Search

Atmospheric Hg is present in different physical and chemical forms, which determine its atmospheric transformation and transport\\u000a capacities. The chemistry of Hg in flue gases is thus of importance for the deposition pattern around point source emissions.\\u000a In order to apply Hg cleaning methods in flue gases its speciation is also of importance. To investigate this under realistic\\u000a conditions, a

B. Hall; P. Schager; O. Lindqvist

1991-01-01

228

Nuclear field shift effect in chemical exchange reactions  

NASA Astrophysics Data System (ADS)

The classic theory of stable isotope fractionation in chemical exchange reactions has been established by Bigeleisen, Mayer, and Urey in 1947. The theory was based on the difference of molecular vibrational energies of isotopomers that are proportional to the respective masses, and hence, results in mass-dependent isotope effect only. In 1996, this conventional mass-dependent theory has been expanded by Bigeleisen to include a mass-independent term named the nuclear field shift effect. The nuclear field shift is an isotope shift in orbital electrons, which results from the isotopic difference in nuclear size and shape. The new equation defined by Bigeleisen (at a constant temperature) can be simply expressed as, ln ? = ? A + (? m/mm') B, where ? is the isotope separation factor, ? isotopic difference in mean-square nuclear charge radius, ?m difference between isotopic masses m and m'. A and B are scaling factors of the nuclear field shift effect and the conventional mass effect, respectively. Since this new theory was presented, the mass-independent isotope fractionation of various elements, e.g, Ti, Cr, Ni, Zn, Sr, Zr, Mo, Ru, Cd, Te, Ba, Nd, Sm, Gd, Yb, and U, found in chemical exchange systems has been successfully explained as the nuclear field shift effect. In our most recent studies, the nuclear field shift effect of Cr, Mo, Ru, Cd, and Te isotopes has been found in laboratory scale experiments. The isotopes of these elements were fractionated by using a liquid-liquid extraction system (a ligand exchange system) at room temperature. The isotopic analysis was performed by the multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) with a typical precision of <100 ppm (at ENS Lyon or UC Davis). Isotope enrichment factors did not show mass-dependent trend, but possessed a similar variation of their nuclear charge radii. For Cr, we tested a different chemical exchange system (a redox system): at high temperature (723-1023 K), an eutectic melt was contacted with a liquid metal. In this system, the nuclear field shift effect of Cr was also found. All these experimental results suggest that the nuclear field shift effect may occur in every chemical exchange reaction at various temperatures to various degrees. Therefore, isotopic anomalies found in a natural system might be partly or largely affected by the nuclear field shift effect via chemical reactions occurred in the nature. In order to clarify the degree and significance of its contribution, we may need to pay more attention to the nuclear field shift effect created chemically.

Fujii, T.; Moynier, F.; Yin, Q.; Albarède, F.

2007-12-01

229

Kinetics and thermodynamics of chemical reactions in Li/SOCl2 cells  

NASA Technical Reports Server (NTRS)

Work is described that was designed to determine the kinetic constants necessary to extrapolate kinetic data on Li/SOCl2 cells over the temperature range from 25 to 75 C. A second objective was to characterize as far as possible the chemical reactions that occur in the cells since these reactions may be important in understanding the potential hazards of these cells. The kinetics of the corrosion processes in undischarged Li/SOCl2 cells were determined and separated according to their occurrence at the anode and cathode; the effects that switching the current on and off has on the corrosion reactions was determined; and the effects of discharge state on the kinetics of the corrosion process were found. A thermodynamic analysis of the current-producing reactions in the cell was done and is included.

Hansen, Lee D.; Frank, Harvey

1987-01-01

230

Review and analysis of high temperature chemical reactions and the effect of non-equilibrium conditions  

NASA Technical Reports Server (NTRS)

Chemical reactions at high temperatures have been considered extensively because of their importance to the heating effects on re-entry of space vehicles. Data on these reactions however, are not abundant and even when found there are discrepancies in data collected by various investigators. In particular, data for recombination reactions are calculated from the dissociation reactions or vice versa through the equilibrium constant. This involves the use of the principle of detailed balancing. This principle is discussed in reference to conditions where it is valid as well as to those where it is not valid. Related topics that merit further study or for which applicable information was available are briefly mentioned in an appendix to this report.

Johnson, R. E.

1986-01-01

231

Computed Potential Energy Surfaces and Minimum Energy Pathway for Chemical Reactions  

NASA Technical Reports Server (NTRS)

Computed potential energy surfaces are often required for computation of such observables as rate constants as a function of temperature, product branching ratios, and other detailed properties. We have found that computation of the stationary points/reaction pathways using CASSCF/derivative methods, followed by use of the internally contracted CI method with the Dunning correlation consistent basis sets to obtain accurate energetics, gives useful results for a number of chemically important systems. Applications to complex reactions leading to NO and soot formation in hydrocarbon combustion are discussed.

Walch, Stephen P.; Langhoff, S. R. (Technical Monitor)

1994-01-01

232

Observation of a chemical reaction using a micromechanical sensor  

NASA Astrophysics Data System (ADS)

We describe a new form of calorimeter designed for use in gaseous and vacuum environments which can sense chemical reactions with an estimated sensitivity limit of ? 1 pJ. The device is based on a micromechanical Si lever coated with a thick layer of Al upon which a sample in the form of a thin layer is fixed or deposited. Heat fluxes are detected by measuring the cantilever deflection induced by the differential thermal expansion of the lever (bimetallic effect) using the optical position sensor from a force microscope. The limit of sensitivity to local temperature changes is ? 10 -5 K at 300 K. Using this technique the catalytic conversion of H 2 + O 2 to form H 2O over a thin Pt overcoated layer is observed to exhibit self-sustained oscillations in the reaction rate on the macroscopic scale.

Gimzewski, J. K.; Gerber, Ch.; Meyer, E.; Schlittler, R. R.

1994-01-01

233

Thermal energy storage. [by means of chemical reactions  

NASA Technical Reports Server (NTRS)

The principles involved in thermal energy storage by sensible heat, chemical potential energy, and latent heat of fusion are examined for the purpose of evolving selection criteria for material candidates in the low ( 0 C) and high ( 100 C) temperature ranges. The examination identifies some unresolved theoretical considerations and permits a preliminary formulation of an energy storage theory. A number of candidates in the low and high temperature ranges are presented along with a rating of candidates or potential candidates. A few interesting candidates in the 0 to 100 C region are also included. It is concluded that storage by means of reactions whose reversibility can be controlled either by product removal or by catalytic means appear to offer appreciable advantages over storage with reactions whose reversability cannot be controlled. Among such advantages are listed higher heat storage capacities and more favorable options regarding temperatures of collection, storage, and delivery. Among the disadvantages are lower storage efficiencies.

Grodzka, P. G.

1975-01-01

234

Detailed chemical kinetic reaction mechanism for oxidation of n-octane and iso-octane  

Microsoft Academic Search

The development of detailed chemical kinetic reaction mechanisms for oxidation of n-octane and iso-octane is described, with emphasis on the factors which are specific to many large hydrocarbon fuel molecules. Elements which are of particular importance are found to include site-specific abstraction of H atoms, radical isomerization of alkyl radicals by internal H atom abstraction, and rapid ..beta..-scission of the

E. I. Axelsson; K. Brezinsky; F. L. Dryer; W. J. Pitz; C. K. Westbrook

1986-01-01

235

Assessment of reaction-rate predictions of a collision-energy approach for chemical reactions in atmospheric flows  

Microsoft Academic Search

A recently proposed approach for the Direct Simulation Monte Carlo (DSMC) method to calculate chemical-reaction rates is assessed for high-temperature atmospheric species. The new DSMC model reproduces measured equilibrium reaction rates without using any macroscopic reaction-rate information. Since it uses only molecular properties, the new model is inherently able to predict reaction rates for arbitrary non-equilibrium conditions. DSMC non-equilibrium reaction

Michail A. Gallis; Ryan Bomar Bond; John Robert Torczynski

2010-01-01

236

Structural cluster analysis of chemical reactions in solution  

NASA Astrophysics Data System (ADS)

We introduce a simple and general approach to the problem of clustering structures from atomic trajectories of chemical reactions in solution. By considering distance metrics which are invariant under permutation of identical atoms or molecules, we demonstrate that it is possible to automatically resolve as distinct structural clusters the configurations corresponding to reactants, products, and transition states, even in presence of atom-exchanges and of hundreds of solvent molecules. Our approach strongly simplifies the analysis of large trajectories and it opens the way to the construction of kinetic network models of activated processes in solution employing the available efficient schemes developed for proteins conformational ensembles.

Gallet, Grégoire A.; Pietrucci, Fabio

2013-08-01

237

Method for detecting pollutants. [through chemical reactions and heat treatment  

NASA Technical Reports Server (NTRS)

A method is described for detecting and measuring trace amounts of pollutants of the group consisting of ozone, nitrogen dioxide, and carbon monoxide in a gaseous environment. A sample organic solid material that will undergo a chemical reaction with the test pollutant is exposed to the test environment and thereafter, when heated in the temperature range of 100-200 C., undergoes chemiluminescence that is measured and recorded as a function of concentration of the test pollutant. The chemiluminescence of the solid organic material is specific to the pollutant being tested.

Rogowski, R. S.; Richards, R. R.; Conway, E. J. (inventors)

1976-01-01

238

Chemical modification of graphene via hyperthermal molecular reaction.  

PubMed

Chemical functionalization of graphene is achieved by hyperthermal reaction with azopyridine molecular ions. The one-step, room temperature process takes place in high vacuum (10(-7) mbar) using an electrospray ion beam deposition (ES-IBD) setup. For ion surface collisions exceeding a threshold kinetic energy of 165 eV, molecular cation beams of 4,4'-azobis(pyridine) covalently attach to chemical vapor deposited (CVD) graphene. A covalent functionalization degree of 3% of the carbon atoms of graphene is reached after 3-5 h of ion exposure of 2 × 10(14) azopyridinium/cm(2) of which 50% bind covalently. This facile approach for the controlled modification of graphene extends the scope of candidate species that would not otherwise react via existing conventional methods. PMID:25185758

Dubey, Girjesh; Urcuyo, Roberto; Abb, Sabine; Rinke, Gordon; Burghard, Marko; Rauschenbach, Stephan; Kern, Klaus

2014-10-01

239

An Overview of Chemical Processes That Damage Cellular DNA: Spontaneous Hydrolysis, Alkylation, and Reactions with Radicals  

PubMed Central

The sequence of heterocyclic bases on the interior of the DNA double helix constitutes the genetic code that drives the operation of all living organisms. With this said, it is not surprising that chemical modification of cellular DNA can have profound biological consequences. Therefore, the organic chemistry of DNA damage is fundamentally important to diverse fields including medicinal chemistry, toxicology, and biotechnology. This review is designed to provide a brief overview of the common types of chemical reactions that lead to DNA damage under physiological conditions. PMID:19757819

Gates, Kent S.

2009-01-01

240

Assessment of reaction-rate predictions of a collision-energy approach for chemical reactions in atmospheric flows.  

SciTech Connect

A recently proposed approach for the Direct Simulation Monte Carlo (DSMC) method to calculate chemical-reaction rates is assessed for high-temperature atmospheric species. The new DSMC model reproduces measured equilibrium reaction rates without using any macroscopic reaction-rate information. Since it uses only molecular properties, the new model is inherently able to predict reaction rates for arbitrary non-equilibrium conditions. DSMC non-equilibrium reaction rates are compared to Park's phenomenological nonequilibrium reaction-rate model, the predominant model for hypersonic-flow-field calculations. For near-equilibrium conditions, Park's model is in good agreement with the DSMC-calculated reaction rates. For far-from-equilibrium conditions, corresponding to a typical shock layer, significant differences can be found. The DSMC predictions are also found to be in very good agreement with measured and calculated non-equilibrium reaction rates, offering strong evidence that this is a viable and reliable technique to predict chemical reaction rates.

Gallis, Michail A.; Bond, Ryan Bomar; Torczynski, John Robert

2010-06-01

241

Detailed Chemical Kinetic Reaction Mechanism for Biodiesel Components Methyl Stearate and Methyl Oleate  

Microsoft Academic Search

New chemical kinetic reaction mechanisms are developed for two of the five major components of biodiesel fuel, methyl stearate and methyl oleate. The mechanisms are produced using existing reaction classes and rules for reaction rates, with additional reaction classes to describe other reactions unique to methyl ester species. Mechanism capabilities were examined by computing fuel\\/air autoignition delay times and comparing

C Naik; C K Westbrook; O Herbinet; W J Pitz; M Mehl

2010-01-01

242

Detailed chemical kinetic reaction mechanism for biodiesel components methyl stearate and methyl oleate  

Microsoft Academic Search

New chemical kinetic reaction mechanisms are developed for two of the five major components of biodiesel fuel, methyl stearate and methyl oleate. The mechanisms are produced using existing reaction classes and rules for reaction rates, with additional reaction classes to describe other reactions unique to methyl ester species. Mechanism capabilities were examined by computing fuel\\/air autoignition delay times and comparing

C. V. Naik; C. K. Westbrook; O. Herbinet; W. J. Pitz; M. Mehl

2011-01-01

243

Stability of Traveling Waves for a Class of Reaction-diffusion Systems that Arise in Chemical Reaction Models  

E-print Network

= concentration of unburned fuel. · = normalized reaction rate. · > 0 is the "exothermicity" parameter. · u = 0 is a background temperature at which the reaction does not take place. #12;3 We are interested in combustionStability of Traveling Waves for a Class of Reaction-diffusion Systems that Arise in Chemical

Schecter, Stephen

244

Irreversible bimolecular chemical reactions on directed scale-free networks.  

PubMed

Kinetics of irreversible bimolecular chemical reactions A+A?0 and A+B?0 on directed scale-free networks with the in-degree distribution P(in)(k)?k(-?)(in) and the out-degree distribution P(out)(?)??(-?)(out) are investigated. Since the correlation between k and ? of each node generally exists in directed networks, we control the correlation (k?) with the probability r?[0,1] by two different algorithms for the construction of the directed networks, i.e., the so-called k and ? algorithms. For r=1, the k algorithm gives (k?)=(k(2)), whereas the ? algorithm gives (k?)=(?(2). For r=0, (k?)=(k)(?) for both algorithms. The kinetics of both reactions are analyzed using heterogeneous mean-field (HMF) theory and Monte Carlo simulations. The density of particles (?) algebraically decays in time t as ?(t)?t(-?). The kinetics of both reactions are determined by the same rate equation, d?/dt=a?(2)+b?(?-1), apart from coefficients. The exponent ? is determined by the algorithm: ?=?(in) for the k algorithm (r?0) and ?=?(min) for the ? algorithm (r>0), where ?(min) is the smaller exponent between ?(in) and ?(out). For ?>3, one observes the ordinary mean-field kinetics, ??1/t (?=1). In contrast, for ?<3, ?(t) anomalously decays with ?=1/(?-2). The HMF predictions are confirmed by the simulations on quenched directed networks. PMID:24229156

Kwon, Sungchul; Kim, Yup

2013-10-01

245

Modeling of Sheath Ion-Molecule Reactions in Plasma Enhanced Chemical Vapor Deposition of Carbon Nanotubes  

NASA Technical Reports Server (NTRS)

In many plasma simulations, ion-molecule reactions are modeled using ion energy independent reaction rate coefficients that are taken from low temperature selected-ion flow tube experiments. Only exothermic or nearly thermoneutral reactions are considered. This is appropriate for plasma applications such as high-density plasma sources in which sheaths are collisionless and ion temperatures 111 the bulk p!asma do not deviate significantly from the gas temperature. However, for applications at high pressure and large sheath voltages, this assumption does not hold as the sheaths are collisional and ions gain significant energy in the sheaths from Joule heating. Ion temperatures and thus reaction rates vary significantly across the discharge, and endothermic reactions become important in the sheaths. One such application is plasma enhanced chemical vapor deposition of carbon nanotubes in which dc discharges are struck at pressures between 1-20 Torr with applied voltages in the range of 500-700 V. The present work investigates The importance of the inclusion of ion energy dependent ion-molecule reaction rates and the role of collision induced dissociation in generating radicals from the feedstock used in carbon nanotube growth.

Hash, David B.; Govindan, T. R.; Meyyappan, M.

2004-01-01

246

A Quantum Chemical Approach to Reactions in Biomolecules  

Microsoft Academic Search

In order to overcome the limitations of conventional molecular mechanics and quantum mechanics studies of model systems, we\\u000a recently proposed a coherent computational scheme, for very large molecules, in which the subsystem that undergoes the most\\u000a important electronic changes is treated by a semi-empirical quantum chemical method, though the rest of the molecule is described\\u000a by a classical force field.

Nathalie Reuter; Michel Loos; Géerald Monard; Alain Cartier; Bernard Maigret; Jean-Louis Rivail

1997-01-01

247

The quantum dynamics of electronically nonadiabatic chemical reactions  

NASA Technical Reports Server (NTRS)

Considerable progress was achieved on the quantum mechanical treatment of electronically nonadiabatic collisions involving energy transfer and chemical reaction in the collision of an electronically excited atom with a molecule. In the first step, a new diabatic representation for the coupled potential energy surfaces was created. A two-state diabatic representation was developed which was designed to realistically reproduce the two lowest adiabatic states of the valence bond model and also to have the following three desirable features: (1) it is more economical to evaluate; (2) it is more portable; and (3) all spline fits are replaced by analytic functions. The new representation consists of a set of two coupled diabatic potential energy surfaces plus a coupling surface. It is suitable for dynamics calculations on both the electronic quenching and reaction processes in collisions of Na(3p2p) with H2. The new two-state representation was obtained by a three-step process from a modified eight-state diatomics-in-molecules (DIM) representation of Blais. The second step required the development of new dynamical methods. A formalism was developed for treating reactions with very general basis functions including electronically excited states. Our formalism is based on the generalized Newton, scattered wave, and outgoing wave variational principles that were used previously for reactive collisions on a single potential energy surface, and it incorporates three new features: (1) the basis functions include electronic degrees of freedom, as required to treat reactions involving electronic excitation and two or more coupled potential energy surfaces; (2) the primitive electronic basis is assumed to be diabatic, and it is not assumed that it diagonalizes the electronic Hamiltonian even asymptotically; and (3) contracted basis functions for vibrational-rotational-orbital degrees of freedom are included in a very general way, similar to previous prescriptions for locally adiabatic functions in various quantum scattering algorithms.

Truhlar, Donald G.

1993-01-01

248

Coupling of hydrologic transport and chemical reactions in a stream affected by acid mine drainage  

USGS Publications Warehouse

Experiments in St. Kevin Gulch, an acid mine drainage stream, examined the coupling of hydrologic transport to chemical reactions affecting metal concentrations. Injection of LiCl as a conservative tracer was used to determine discharge and residence time along a 1497-m reach. Transport of metals downstream from inflows of acidic, metal-rich water was evaluated based on synoptic samples of metal concentrations and the hydrologic characteristics of the stream. Transport of SO4 and Mn was generally conservative, but in the subreaches most affected by acidic inflows, transport was reactive. Both 0.1-??m filtered and particulate Fe were reactive over most of the stream reach. Filtered Al partitioned to the particulate phase in response to high instream concentrations. Simulations that accounted for the removal of SO4, Mn, Fe, and Al with first-order reactions reproduced the steady-state profiles. The calculated rate constants for net removal used in the simulations embody several processes that occur on a stream-reach scale. The comparison between rates of hydrologie transport and chemical reactions indicates that reactions are only important over short distances in the stream near the acidic inflows, where reactions occur on a comparable time scale with hydrologic transport and thus affect metal concentrations.

Kimball, B.A.; Broshears, R.E.; Bencala, K.E.; McKnight, D.M.

1994-01-01

249

Chemical fibres — An important factor in economic development  

Microsoft Academic Search

1997 set a record for the past 20 years in the growth rate of world production of chemical fibres and filaments. In comparison\\u000a to 1996, this figure, was 11%, primarily due to synthetic fibres, where polyester fibres still occupy the leading position\\u000a (approximately 15 million tons). Production of cellulose (viscose, acetate, cuprammonium) fibres and filaments dropped, especially\\u000a for industrial applications.

E. M. Aizenshtein

1998-01-01

250

Diffusion of Weak Acids across Lipid Bilayer Membranes: Effects of Chemical Reactions in the Unstirred Layers  

Microsoft Academic Search

Chemical reactions in the aqueous unstirred layers of solution adjacent to a membrane can have dramatic effects on the diffusion of solutes across that membrane. This is demonstrated by the diffusion of labeled salicylate and salicylic acid across a phospholipid bilayer membrane. Two types of chemical reactions are considered. The first is an isotopic exchange reaction between the ionic and

John Gutknecht; D. C. Tosteson

1973-01-01

251

Progression in High School Students' (Aged 16-18) Conceptualizations about Chemical Reactions in Solution.  

ERIC Educational Resources Information Center

Explores the development over time of students' understandings of the concept of chemical reaction in the context of two familiar reactions in solution. Based on interviews (n=48), results show that students made some progress in their understanding of the concept of chemical reaction but some fundamental misconceptions remained. (Author/MM)

Boo, Hong-Kwen; Watson, J. R.

2001-01-01

252

Comparison of the efficiency of a thermo-chemical process to that of a fuel cell process when both involve the same chemical reaction  

E-print Network

with the same reactants. A theoretical process is developed to convert heat liberated from a chemical reaction to work. The hypothetical process is carried over a series of isothermal chemical reactor - heat engine combinations. Conducting the chemical reaction...

Bulusu, Seshu Periah

2009-05-15

253

The Role of Comprehensive Detailed Chemical Kinetic Reaction Mechanisms in Combustion Research  

Microsoft Academic Search

Recent developments by the authors in the field of comprehensive detailed chemical kinetic reaction mechanisms for hydrocarbon fuels are reviewed. Examples are given of how these mechanisms provide fundamental chemical insights into a range of combustion applications. Practical combustion consists primarily of chemical heat release from reactions between a fuel and an oxidizer, and computer simulations of practical combustion systems

C K Westbrook; W J Pitz; H J Curran; M Mehl

2008-01-01

254

Molecular dynamics simulations of a chemical reaction; conditions for local equilibrium in a temperature gradient  

E-print Network

Molecular dynamics simulations of a chemical reaction; conditions for local equilibrium as in local chemical equilibrium (DrG = 0) in the supercritical fluid, for temperature gradients up to 1012 K in the hot region. 1. Introduction A chemical reaction that occurs far from global equilibrium, has a rate

Kjelstrup, Signe

255

Lyapunov functions, stationary distributions, and non-equilibrium potential for chemical reaction networks  

E-print Network

Lyapunov functions, stationary distributions, and non-equilibrium potential for chemical reaction the well known Lyapunov function of chemical reaction network theory as a scaling limit of the non-equilibrium Abstract We consider the relationship between stationary distributions for stochastic models of chemical

Anderson, David F.

256

Non-invasive estimation of dissipation from non-equilibrium fluctuations in chemical reactions  

E-print Network

Non-invasive estimation of dissipation from non-equilibrium fluctuations in chemical reactions S JOURNAL OF CHEMICAL PHYSICS 139, 124109 (2013) Non-invasive estimation of dissipation from non-equilibrium fluctuations in chemical reactions S. Muy,1 A. Kundu,2 and D. Lacoste1 1 Laboratoire de Physico

Lacoste, David

257

Kinetics of Soil Chemical Reactions: Relationships between Empirical Equations and Diffusion Models  

E-print Network

Kinetics of Soil Chemical Reactions: Relationships between Empirical Equations and Diffusion Models equation has been used by several workers to describe soil chemical reaction rates (Chien and Clayton, 1980-diffusion equa- tions have been used to describe the kinetics of soil chemical processes. Often, several

Sparks, Donald L.

258

Chemical Reactions in the Processing of Mosi2 + Carbon Compacts  

NASA Technical Reports Server (NTRS)

Hot-pressing of MoSi2 powders with carbon at high temperatures reduces the siliceous grain boundary phase in the resultant compact. The chemical reactions in this process were examined using the Knudsen cell technique. A 2.3 wt pct oxygen MoSi2 powder and a 0.59 wt pct oxygen MoSi2 powder, both with additions of 2 wt pct carbon, were examined. The reduction of the siliceous grain boundary phase was examined at 1350 K and the resultant P(SiO)/P(CO) ratios interpreted in terms of the SiO(g) and CO(g) isobars on the Si-C-O predominance diagram. The MoSi2 + carbon mixtures were then heated at the hot-pressing temperature of 2100 K. Large weight losses were observed and could be correlated with the formation of a low-melting eutectic and the formation and vaporization of SiC.

Jacobson, Nathan S.; Lee, Kang N.; Maloy, Stuart A.; Heuer, Arthur H.

1993-01-01

259

Simulation of quantum interference by reactions of chemical type  

E-print Network

A quantum unitary evolution alternated with measurements is simulated by a bubble filled with fictitious particles called amplitude quanta that move chaotically and can be transformed by the simple rules that look like chemical reactions. A basic state of simulated system is treated as a collision of the two corresponding amplitude quanta, that gives the quantum statistics of measurements. The movement of the external membrane of the bubble corresponds to the classical dynamics of the simulated system. Measurements are treated as the membrane perforations and they are completely determined by initial conditions. An identity of particles and an entanglement is simulated by the membranes touching. The required memory grows linearly where the number of particles increases, but entangled states of the big number of particles can be simulated. The method can be used for a visualization of quantum dynamics.

Yuri Ozhigov

2003-11-06

260

HSAB principle applied to the time evolution of chemical reactions.  

PubMed

Time evolution of various reactivity parameters such as electronegativity, hardness, and polarizability associated with a collision process between a proton and an X- atom/ion (X = He, Li(+), Be(2+), B(3+), C(4+)) in its ground ((1)S) and excited((1)P,(1)D,(1)F) electronic states as well as various complexions of a two-state ensemble is studied using time-dependent and excited-state density functional theory. This collision process may be considered to be a model mimicking the actual chemical reaction between an X-atom/ion and a proton to give rise to an XH(+) molecule. A favorable dynamical process is characterized by maximum hardness and minimum polarizability values according to the dynamical variants of the principles of maximum hardness and minimum polarizability. An electronic excitation or an increase in the excited-state contribution in a two-state ensemble makes the system softer and more polarizable, and the proton, being a hard acid, gradually prefers less to interact with X as has been discerned through the drop in maximum hardness value and the increase in the minimum polarizability value when the actual chemical process occurs. Among the noble gas elements, Xe is the most reactive. During the reaction: H(2) + H(+) --> H(3)(+) hardness maximizes and polarizability minimizes and H(2) is more reactive in its excited state. Regioselectivity of proton attack in the O-site of CO is clearly delineated wherein HOC(+) may eventually rearrange itself to go to the thermodynamically more stable HCO(+). PMID:12603158

Chattaraj, Pratim Kumar; Maiti, Buddhadev

2003-03-01

261

Chemical reactions of excited nitrogen atoms for short wavelength chemical lasers. Final technical report  

SciTech Connect

Accomplishments of this program include the following: (1) Scalable, chemical generation of oxygen atoms by reaction of fluorine atoms and water vapor. (2) Production of nitrogen atom densities of 1 {times} 10{sup 1}5 cm{sup {minus}3} with 5% electrical efficiency by injecting trace amounts of fluorine into microwave discharged nitrogen. (3) Production of cyanide radicals by reaction of high densities of N atoms with cyanogen. (4) Production of carbon atoms by reaction of nitrogen atoms with cyanogen or with fluorine atoms and hydrogen cyanide. (5) Confirmation that the reaction of carbon atoms and carbonyl sulfide produces CS(a{sup 3} {Pi}{sub r}), as predicted by conservation of electron spin and orbital angular momenta and as proposed by others under another SWCL program. (6) Production of cyanide radicals by injection of cyanogen halides into active nitrogen and use as spectroscopic calibration source. (7) Demonstration that sodium atoms react with cyanogen chloride, bromide and iodide and with cyanuric trifluoride to produce cyanide radicals. (8) Demonstration of the potential utility of the fluorine atom plus ammonia reaction system in the production of NF(b{sup l}{Sigma}{sup +}) via N({sup 2}D) + F{sub 2}.

Not Available

1989-12-15

262

Mathematically Reduced Chemical Reaction Mechanism Using Neural Networks  

SciTech Connect

This is the final technical report for the project titled 'Mathematically Reduced Chemical Reaction Mechanism Using Neural Networks'. The aim of the project was to develop an efficient chemistry model for combustion simulations. The reduced chemistry model was developed mathematically without the need of having extensive knowledge of the chemistry involved. To aid in the development of the model, Neural Networks (NN) was used via a new network topology known as Non-linear Principal Components Analysis (NPCA). A commonly used Multilayer Perceptron Neural Network (MLP-NN) was modified to implement NPCA-NN. The training rate of NPCA-NN was improved with the GEneralized Regression Neural Network (GRNN) based on kernel smoothing techniques. Kernel smoothing provides a simple way of finding structure in data set without the imposition of a parametric model. The trajectory data of the reaction mechanism was generated based on the optimization techniques of genetic algorithm (GA). The NPCA-NN algorithm was then used for the reduction of Dimethyl Ether (DME) mechanism. DME is a recently discovered fuel made from natural gas, (and other feedstock such as coal, biomass, and urban wastes) which can be used in compression ignition engines as a substitute for diesel. An in-house two-dimensional Computational Fluid Dynamics (CFD) code was developed based on Meshfree technique and time marching solution algorithm. The project also provided valuable research experience to two graduate students.

Ziaul Huque

2007-08-31

263

Electrokinetics for control of on-chip chemical reactions.  

NASA Astrophysics Data System (ADS)

It is well known that electrokinetics affords precise control over flow and species transport in microfluidic systems through simple manipulation of externally applied electric potentials. In this work it is demonstrated how electrokinetic effects can be extended to provide simultaneous control over on-chip chemical reactions through manipulation of the local thermal (ohmic/joule heating), shear (electroosmosis) and electrical (electrophoresis) energies at the reaction site. The coupling of the electrical, flow and ``whole-chip'' thermal effects in both the fluidic and substrate domains are investigated through extensive finite element simulations and experimentally validated using microscale fluorescence thermometry. The simulations reveal changes in viscosity and local conductivity on the order of 50% induced by changes in the fluidic geometry. General chip design guidelines for maximizing or minimizing these effects will also be discussed. The degree of precision available and clinical utility of the technique is demonstrated through the detection of a single base pair mutation (single nucleotide polymorphism) in a DNA microarray integrated into a PDMS/glass microfluidic chip.

Erickson, David; Venditti, Roberto

2005-03-01

264

Changes in mechanical property of sandstone by chemical reaction  

NASA Astrophysics Data System (ADS)

The effect of increasing temperature on the hydro-mechanical properties of rocks is a highly important topic. When we select a site for underground excavation for the disposal of high-level radioactive waste and CO2-underground storage, it is necessary to know the effects of temperature and confining pressure with water, called the deep geological environment. In fact increasing temperatures cause micro-cracking and the production of new clay minerals under long-term immersion in hot water at 90°E C (Suzuki et al., 1998). It has to be pointed out that the long-term stability of rocks under deep geological environments can be damaged by chemical processes. Such chemical weathering in hot water is a time-dependent phenomenon, and it is important to consider hydro-thermal-mechanical properties in rocks under deep geological environments. However, there are few studies of the time and temperature effects on mechanical properties. Additionally, there are only a few studies of sedimentary rocks compared with crystalline rocks. With this objective in mind, Shirahama sandstones were immersed in 90°E C water for 500 days. Using samples damaged by chemical weathering, triaxial compression tests were preformed under a 10MPa confining pressure. In addition, the microstructure of damaged samples was observed using a secondary electron microscope (SEM), and we measured porosity using a mercury injection porosimetry method (MIP) in order to evaluate the damaged sample. In order to study the properties of sedimentary rocks under deep geological environments, laboratory chemical weathering tests were carried out by immersing Shirahama sandstone in 90°E hot water for 100 and 500 days. Using that immersed sample, we carried out mechanical tests, measuring porosity and made microstructural observations We have the following conclusions: 1) The mechanical properties, strength, secant modulus E10 and tangential modulus E50, decrease with the time of soaking in hot water. E10, especially, is much reduced by chemical weathering. 2) New clay minerals produced by chemical weathering can be observed not only in the matrix portion but also in the micropores so as to fill in space.

Takemura, T.; Manaka, M.; Takahashi, M.

2006-12-01

265

Chemical Characterization of Secondary Organic Aerosol Formed from Atmospheric Aqueous-phase Reactions of Phenolic Compounds  

NASA Astrophysics Data System (ADS)

Phenolic compounds, which are released in significant amounts from biomass burning, may undergo fast aqueous-phase reactions to form secondary organic aerosol (SOA) in the atmosphere. Understanding the aqueous-phase reaction mechanisms of these compounds and the composition of their reaction products is thus important for constraining SOA sources and predicting organic aerosol properties in models. In this study, we investigate the aqueous-phase reactions of three phenols (phenol, guaiacol and syringol) with two oxidants - excited triplet states (3C*) of non-phenolic aromatic carbonyls and hydroxyl radical (OH). By employing four analytical methods including high-resolution aerosol mass spectrometry, total organic carbon analysis, ion chromatography, and liquid chromatography-mass spectrometry, we thoroughly characterize the chemical compositions of the low volatility reaction products of phenols and propose formation mechanisms based on this information. Our results indicate that phenolic SOA is highly oxygenated, with O/C ratios in the range of 0.83-1.03, and that the SOA of phenol is usually more oxidized than those of guaiacol and syringol. Among the three precursors, syringol generates the largest fraction of higher molecular weight (MW) products. For the same precursor, the SOA formed via reaction with 3C* is less oxidized than that formed via reaction with OH. In addition, oxidation by 3C* enhances the formation of higher MW species, including phenolic dimers, higher oligomers and hydroxylated products, compared to reactions initiated by OH, which appear to favor the formation of organic acids. However, our results indicate that the yields of small organic acids (e.g., formate, acetate, oxalate, and malate) are low for both reaction pathways, together accounting for less than 5% of total SOA mass.

Yu, L.; Smith, J.; Anastasio, C.; Zhang, Q.

2012-12-01

266

Chemical Reaction and Flow Modeling in Fullerene and Nanotube Production  

NASA Technical Reports Server (NTRS)

The development of processes to produce fullerenes and carbon nanotubes has largely been empirical. Fullerenes were first discovered in the soot produced by laser ablation of graphite [1]and then in the soot of electric arc evaporated carbon. Techniques and conditions for producing larger and larger quantities of fullerenes depended mainly on trial and error empirical variations of these processes, with attempts to scale them up by using larger electrodes and targets and higher power. Various concepts of how fullerenes and carbon nanotubes were formed were put forth, but very little was done based on chemical kinetics of the reactions. This was mainly due to the complex mixture of species and complex nature of conditions in the reactors. Temperatures in the reactors varied from several thousand degrees Kelvin down to near room temperature. There are hundreds of species possible, ranging from atomic carbon to large clusters of carbonaceous soot, and metallic catalyst atoms to metal clusters, to complexes of metals and carbon. Most of the chemical kinetics of the reactions and the thermodynamic properties of clusters and complexes have only been approximated. In addition, flow conditions in the reactors are transient or unsteady, and three dimensional, with steep spatial gradients of temperature and species concentrations. All these factors make computational simulations of reactors very complex and challenging. This article addresses the development of the chemical reaction involved in fullerene production and extends this to production of carbon nanotubes by the laser ablation/oven process and by the electric arc evaporation process. In addition, the high-pressure carbon monoxide (HiPco) process is discussed. The article is in several parts. The first one addresses the thermochemical aspects of modeling; and considers the development of chemical rate equations, estimates of reaction rates, and thermodynamic properties where they are available. The second part addresses modeling of the arc process for fullerene and carbon nanotube production using O-D, 1-D and 2-D fluid flow models. The third part addresses simulations of the pulsed laser ablation process using time-dependent techniques in 2-D, and a steady state 2-D simulation of a continuous laser ablation process. The fourth part addresses steady state modeling in O-D and 2-D of the HiPco process. In each of the simulations, there is a variety of simplifications that are made that enable one to concentrate on one aspect or another of the process. There are simplifications that can be made to the chemical reaction models , e.g. reduction in number of species by lumping some of them together in a representative species. Other simulations are carried out by eliminating the chemistry altogether in order to concentrate on the fluid dynamics. When solving problems with a large number of species in more than one spatial dimension, it is almost imperative that the problem be decoupled by solving for the fluid dynamics to find the fluid motion and temperature history of "particles" of fluid moving through a reactor. Then one can solve the chemical rate equations with complex chemistry following the temperature and pressure history. One difficulty is that often mixing with an ambient gas is involved. Therefore, one needs to take dilution and mixing into account. This changes the ratio of carbon species to background gas. Commercially available codes may have no provision for including dilution as part of the input. One must the write special solvers for including dilution in decoupled problems. The article addresses both ful1erene production and single-walled carbon nanotube (SWNT) production. There are at least two schemes or concepts of SWNT growth. This article will only address growth in the gas phase by carbon and catalyst cluster growth and SW T formation by the addition of carbon. There are other models that conceive of SWNT growth as a phase separation process from clusters me up carbon and metal catalyst, with the carbon precipitating from the cluster as it co

Scott, Carl D.; Farhat, Samir; Greendyke, Robert B.

2004-01-01

267

Detailed Chemical Kinetic Reaction Mechanisms for Autoignition of Isomers of Heptane Under Rapid Compression  

Microsoft Academic Search

Detailed chemical kinetic reaction mechanisms are developed for combustion of all nine isomers of heptane (CH), and these mechanisms are tested by simulating autoignition of each isomer under rapid compression machine conditions. The reaction mechanisms focus on the manner in which the molecular structure of each isomer determines the rates and product distributions of possible classes of reactions. The reaction

C K Westbrook; W J Pitz; J E Boercker; H J Curran; J F Griffiths; C Mohamed; M Ribaucour

2001-01-01

268

Enzyme catalysis Enzyme catalysis is the catalysis of chemical reactions by  

E-print Network

Enzyme catalysis Enzyme catalysis is the catalysis of chemical reactions by specialized proteins known as enzymes. Catalysis of biochemical reactions in the cell is vital due to the very low reaction rates of the uncatalysed reactions. The mechanism of enzyme catalysis is similar in principle to other

Cavanagh, John

269

Using chemical tracers in hillslope soils to estimate the importance of chemical denudation under  

E-print Network

from weathered bedrock or saprolite through a variety of processes such as penetration by tree roots is developed; the two phases are a chemically immobile phase, which has far lower solubility than the bulk soil be removed from the system through chemical weathering. Chemical denudation rates in hillslope soils can

270

Extended Hartree-Fock theory of chemical reactions. VIII. Hydroxylation reactions by P450  

NASA Astrophysics Data System (ADS)

We have investigated the reaction pathways for the primary hydroxylation reaction of trimethylmethane by a high-valent Fe(IV) dbond O porphyrin pi-cation radical species known as compound I at the B3LYP/CEP-31G level. The isoelectronic analogy of the Fe(IV) dbond O core of compound I to a molecular oxygen (O2) has been successfully used to clarify the important roles of the singlet excited state of the Fe(IV) dbond O core in the alkane hydroxylation, which has hitherto been neglected. The reaction is initiated by the rate-determining hydrogen-atom abstraction from the substrate to give a discrete radical intermediate complex, in accordance with the conventional radical rebound mechanism. Similar to the chemistry of O2, however, one of the singlet excited states, i.e., the diradical component of the 1Delta state of the Fe(IV) dbond O core intercepts the triplet ground state (the 3Sigma state) in the region of the transition state for the hydrogen abstraction. Our findings strongly indicate that the exchange polarization or intersystem crossing for the nonradiative transition to the locally singlet state is highly important to enhance the reactivity of compound I.

Isobe, Hiroshi; Nishihara, Satomichi; Shoji, Mitsuo; Yamanaka, Shusuke; Shimada, Jiro; Hagiwara, Masayuki; Yamaguchi, Kizashi

271

Measurements and analysis of alpha-induced reactions of importance for nuclear astrophysics  

NASA Astrophysics Data System (ADS)

Reactions during stellar helium burning are of primary importance for understanding nucleosynthesis. A detailed understanding of the critical reaction chain 4He(2alpha, gamma)12C( alpha, gamma)16O(alpha, gamma) 20Ne is necessary both because it is the primary energy source and because it determines the ratio of 12C to 16O produced, which in turn significantly effects subsequent nucleosynthesis. Also during Helium burning, the reactions 22Ne(alpha, n)25Mg and 22Ne(alpha, gamma )26Mg are crucial in determining the amount of neutrons available for the astrophysical s-process. This thesis presents new experimental results concerning the 16O(alpha, gamma) 20Ne, 22Ne(alpha, n)25Mg, and 22Ne(alpha, gamma)26Mg reaction rates. These results are then applied to the calculation of the associated stellar reaction rates in order to achieve better accuracy.

de Messieres, Genevieve Escande

2011-11-01

272

Kinetics of Thermochemical Reactions Important in the Venus Atmospheric Sulfur Cycle  

NASA Technical Reports Server (NTRS)

The purpose of this project was to experimentally measure the rates of several thermochemical gas-solid reactions between sulfur gases in the Venus atmosphere and reactive minerals on the hot Venus surface. Despite the great importance of these reactions for the maintenance of significant amounts of sulfur gases (and thus for the maintenance of the global cloud cover) in the atmosphere of Venus, essentially no kinetic data are currently available for them.

Fegley, Bruce, Jr.

1997-01-01

273

Direct Monte Carlo simulation of chemical reaction systems: Internal energy transfer and an energy-dependent unimolecular reaction  

E-print Network

Direct Monte Carlo simulation of chemical reaction systems: Internal energy transfer and an energy a direct Monte Carlo simulation of an energy-dependent t&molecular reaction system of the type A+ B can be treated by Monte Carlo simulations. One of the most useful methods is Bird's direct simulation

Anderson, James B.

274

Computed Potential Energy Surfaces and Minimum Energy Pathways for Chemical Reactions  

NASA Technical Reports Server (NTRS)

Computed potential energy surfaces are often required for computation of such parameters as rate constants as a function of temperature, product branching ratios, and other detailed properties. For some dynamics methods, global potential energy surfaces are required. In this case, it is necessary to obtain the energy at a complete sampling of all the possible arrangements of the nuclei, which are energetically accessible, and then a fitting function must be obtained to interpolate between the computed points. In other cases, characterization of the stationary points and the reaction pathway connecting them is sufficient. These properties may be readily obtained using analytical derivative methods. We have found that computation of the stationary points/reaction pathways using CASSCF/derivative methods, followed by use of the internally contracted CI method to obtain accurate energetics, gives usefull results for a number of chemically important systems. The talk will focus on a number of applications including global potential energy surfaces, H + O2, H + N2, O(3p) + H2, and reaction pathways for complex reactions, including reactions leading to NO and soot formation in hydrocarbon combustion.

Walch, Stephen P.; Langhoff, S. R. (Technical Monitor)

1994-01-01

275

Mathematically Reduced Chemical Reaction Mechanism Using Neural Networks  

SciTech Connect

This is an annual technical report for the work done over the last year (period ending 9/30/2005) on the project titled ''Mathematically Reduced Chemical Reaction Mechanism Using Neural Networks''. The aim of the project is to develop an efficient chemistry model for combustion simulations. The reduced chemistry model will be developed mathematically without the need of having extensive knowledge of the chemistry involved. To aid in the development of the model, Neural Networks (NN) will be used via a new network topology know as Non-linear Principal Components Analysis (NPCA). We report on the development of a novel procedure to speed up the training of NPCA. The same procedure termed L{sub 2}Boost can be used to increase the order of approximation of the Generalized Regression Neural Network (GRNN). It is pointed out that GRNN is a basic procedure for the emerging mesh free CFD. Also reported is an efficient simple approach of computing the derivatives of GRNN function approximation using complex variables or the Complex Step Method (CSM). The results presented demonstrate the significance of the methods developed and will be useful in many areas of applied science and engineering.

Nelson Butuk

2005-12-01

276

Mathematically Reduced Chemical Reaction Mechanism Using Neural Networks  

SciTech Connect

This is an annual technical report for the work done over the last year (period ending 9/30/2004) on the project titled ''Mathematically Reduced Chemical Reaction Mechanism Using Neural Networks''. The aim of the project is to develop an efficient chemistry model for combustion simulations. The reduced chemistry model will be developed mathematically without the need of having extensive knowledge of the chemistry involved. To aid in the development of the model, Neural Networks (NN) will be used via a new network topology know as Non-linear Principal Components Analysis (NPCA). We report on the development of a procedure to speed up the training of NPCA. The developed procedure is based on the non-parametric statistical technique of kernel smoothing. When this smoothing technique is implemented as a Neural Network, It is know as Generalized Regression Neural Network (GRNN). We present results of implementing GRNN on a test problem. In addition, we present results of an in house developed 2-D CFD code that will be used through out the project period.

Nelson Butuk

2004-12-01

277

Chemical reactions between Venus' surface and atmosphere - An update. (Invited)  

NASA Astrophysics Data System (ADS)

The surface of Venus, at ~740K, is hot enough to allow relatively rapid chemical reactions between it and the atmosphere, i.e. weathering. Venus chemical weathering has been explored in detail [1], to the limits of available data. New data from Venus Express (VEx) and new ideas from exoplanets have sparked a modest renewal of interest in Venus weathering. Venus' surface cannot be observed in visible light, but there are several NIR ';windows' through its atmosphere that allow surface imaging. The VIRTIS spectrometer on VEx viewed the surface through one window [2]; emissivity variations among lava flows on Imdr and Themis Regios have been explained as varying degrees of weathering, and thus age [3]. The VMC camera on VEx also provides images through a NIR window, which suggest variable degrees of weathering on some basaltic plains [4]. Indirect evidence for weathering may come from varying SO2 abundance at Venus' cloud tops; repeated rapid increases and gradual declines may represent volcanic eruptions followed by weathering to form sulfate minerals [5]. Continued geochemical modeling relevant to Venus weathering is motivated by expolanet studies [6]. Models have been extended to hypothetical exo-Venuses of different temperatures and surface compositions [7]. The idea that Venus' atmosphere composition can be buffered by reaction with its surface was explored in detail, and the derived constraint extended to other types of planets [8]. Several laboratories are investigating Venus weathering, motivated in part by the hope that they can provide real constraints on timescales of Venus volcanism [3]. Aveline et al. [9] are extending early studies [10] by reacting rocks and minerals with concentrated SO2 (to accelerate reaction rates to allow detectability of products). Kohler et al. [11] are investigating the stability of metals and chalcogenides as possible causes of the low-emissivity surfaces at high elevations. Berger and Aigouy [12] studied rock alteration on a hypothetical early Venus with a water-rich atmosphere. Martin et al. [13] investigated the fate of weathered rock when heated (by igneous or impact events). Our understanding of Venus' geological history is stymied by a lack of data - spacecraft observations of and/or at its surface. VMC on VEx may continue to provide new data on surface emissivity, but their interpretation is inherently ambiguous. Laboratory experiments seem the most promising approach - attempting to quantify rates of weathering and thus volcanism [3], and (with luck) framing significant problems that can be directly answered by spacecraft observations. [1] Fegley B.Jr. et al. (1997) In Venus II. U. Ariz. Press. p. 591. [2] Helbert J. et al. (2008) GRL 35, L11201. [3] Smrekar S.E et al. (2010) Science 328, 605-608. [4] Basilevsky A.T. et al. (2012) Icarus 217, 434-450. [5] Marcq E. et al. (2013) Nature Geoscience 6, 25-28. [6] Kane S.R. et al. (2013) Astrophysical J. 770, L20. [7] Schaefer L. & Fegley B.Jr. (2011) Astrophysical J. 729, 6. [8] Treiman A.H. & Bullock M.A. (2012) Icarus 217, 534-541. [9] Aveline D.C. et al. (2011) Lunar Planet. Sci. Conf. 42, Abstr. #2165. [10] Fegley B.Jr. & Prinn R.G. (1989) Nature 337, 55-58. [11] Kohler E. et al. (2012) Lunar Planet. Sci. Conf. 43, Abstr. #2749. [12] Berger G. & Aigouy T. (2011) Lunar Planet. Sci. Conf. 42, Abstr. #1660. [13] Martin A.M. et al. (2012) Earth Planet. Sci. Lett. 331-332, 291-304.

Treiman, A. H.

2013-12-01

278

The importance of nanocrystal precursor conversion kinetics: mechanism of the reaction between cadmium carboxylate and cadmium bis(diphenyldithiophosphinate).  

PubMed

We describe the synthesis of cadmium bis(diphenyldithiophosphinate) (Cd(S(2)PPh(2))(2)) from secondary phosphine sulfides and its conversion to cadmium sulfide nanocrystals. Heating Cd(S(2)PPh(2))(2) and cadmium tetradecanoate (?4 equiv) to 240 °C results in complete conversion of Cd(S(2)PPh(2))(2) to cadmium sulfide nanocrystals with tetradecanoate surface termination. The nanocrystals have a narrow size distribution (d = 3.8-4.1 nm, ? < 10%) that is evident from the line width of the lowest energy absorption feature (? = 412-422 nm, fwhm = 0.17 eV) and display bright photoluminescence (PLQY(band edge+trap) = 36%). Interestingly, the final diameter is insensitive to the reaction conditions, including the total concentration of precursors and initial cadmium to sulfur ratio. Monitoring the reaction with (31)P NMR, UV-visible, and infrared absorption spectroscopies shows that the production of cadmium diphenylphosphinate (Cd(O(2)PPh(2))(2)) and tetradecanoic anhydride co-products is coupled with the formation of cadmium sulfide. From these measurements we propose a balanced chemical equation for the conversion reaction and use it to optimize a synthesis that affords CdS nanocrystals in quantitative yield. In light of these results we discuss the importance of well-defined precursor reactivity to reproducible conversion kinetics and the synthesis of nanocrystals with unambiguous chemical composition. PMID:23043371

Hendricks, Mark P; Cossairt, Brandi M; Owen, Jonathan S

2012-11-27

279

Studies in photochemical smog chemistry. 1. Atmospheric chemistry of toulene. 2. Analysis of chemical reaction mechanisms for photochemical smog  

SciTech Connect

This study focuses on two related topics in the gas phase organic chemistry of importance in urban air pollution. An experimental effort aimed at developing a new explicit reaction mechanism for the atmospheric photooxidation of toluene is described. This mechanism is tested using experimental data from both indoor and outdoor smog chamber facilities. The predictions of the new reaction mechanism are found to be in good agreement with both sets of experimental data. Additional simulations performed with the new mechanism are used to investigate various mechanistic paths. A theoretical analysis of lumped chemical reaction mechanisms for photochemical smog is presented. Included is a description of a new counter species analysis technique which can be used to analyze any complex chemical reaction mechanism. When applied to mechanisms for photochemical smog, this analysis is shown capable of providing answers to previously inaccessible questions such as the relative contributions of individual organics to photochemical ozone formation.

Leone, J.A.

1985-01-01

280

Method of operating a thermal engine powered by a chemical reaction  

DOEpatents

The invention involves a novel method of increasing the efficiency of a thermal engine. Heat is generated by a non-linear chemical reaction of reactants, said heat being transferred to a thermal engine such as Rankine cycle power plant. The novel method includes externally perturbing one or more of the thermodynamic variables of said non-linear chemical reaction.

Ross, John (Stanford, CA); Escher, Claus (Nieder-Ronstadt, DE)

1988-01-01

281

Detailed Chemical Kinetic Reaction Mechanisms for Combustion of Isomers of Heptane  

Microsoft Academic Search

Detailed chemical kinetic reaction mechanisms are developed for all nine chemical isomers of heptane (CH), following techniques and models developed previously for other smaller alkane hydrocarbon species. These reaction mechanisms are tested at high temperatures by computing shock tube ignition delay times and at lower temperatures by simulating ignition in a rapid compression machine. Although the corresponding experiments have not

C K Westbrook; W J Pitz; H C Curran; J Boercker; E Kunrath

2001-01-01

282

Detailed Chemical Kinetic Reaction Mechanisms for Incineration of Organophosphorus and Fluoro-Organophosphorus Compounds  

Microsoft Academic Search

A detailed chemical kinetic reaction mechanism is developed to describe incineration of the chemical warfare nerve agent sarin (GB), based on commonly used principles of bond additivity and hierarchical reaction mechanisms. The mechanism is based on previous kinetic models of organophosphorus compounds such as TMP, DMMP and DIMP that are often used as surrogates to predict incineration of GB. Kinetic

P A Glaude; C Melius; W J Pitz; C K Westbrook

2001-01-01

283

State-Estimators for Chemical Reaction Networks of Feinberg-Horn-Jackson Zero Deficiency Type  

E-print Network

State-Estimators for Chemical Reaction Networks of Feinberg-Horn-Jackson Zero Deficiency Type for detectability for chemical reaction networks of the Feinberg-Horn-Jackson zero deficiency type. Under introduced by Feinberg, Horn, and Jackson in [6, 7, 8, 9, 11, 12]. As outputs, we take a subset of state

Sontag, Eduardo

284

State-Estimators for Chemical Reaction Networks of Feinberg-Horn-Jackson Zero De ciency Type  

E-print Network

State-Estimators for Chemical Reaction Networks of Feinberg-Horn-Jackson Zero De#12;ciency Type #3 for detectability for chemical reaction networks of the Feinberg-Horn-Jackson zero de#12;ciency type. Under introduced by Feinberg, Horn, and Jackson in [6, 7, 8, 9, 10, 11]. As outputs, we take a subset of state

Chaves, Madalena

285

Detailed chemical kinetic reaction mechanisms for soy and rapeseed biodiesel fuels  

E-print Network

Detailed chemical kinetic reaction mechanisms for soy and rapeseed biodiesel fuels C.K. Westbrooka chemical kinetic reaction mechanism is developed for the five major components of soy biodiesel and rapeseed biodiesel fuels. These components, methyl stearate, methyl oleate, methyl linoleate, methyl

Paris-Sud XI, Université de

286

Acid-Base Chemistry According to Robert Boyle: Chemical Reactions in Words as well as Symbols  

ERIC Educational Resources Information Center

Examples of acid-base reactions from Robert Boyle's "The Sceptical Chemist" are used to illustrate the rich information content of chemical equations. Boyle required lengthy passages of florid language to describe the same reaction that can be done quite simply with a chemical equation. Reading or hearing the words, however, enriches the student's…

Goodney, David E.

2006-01-01

287

Achieving Chemical Equilibrium: The Role of Imposed Conditions in the Ammonia Formation Reaction  

ERIC Educational Resources Information Center

Under conditions of constant temperature T and pressure P, chemical equilibrium occurs in a closed system (fixed mass) when the Gibbs free energy G of the reaction mixture is minimized. However, when chemical reactions occur under other conditions, other thermodynamic functions are minimized or maximized. For processes at constant T and volume V,…

Tellinghuisen, Joel

2006-01-01

288

Design criteria for extraction with chemical reaction and liquid membrane permeation  

NASA Technical Reports Server (NTRS)

The design criteria for heterogeneous chemical reactions in liquid/liquid systems formally correspond to those of classical physical extraction. More complex models are presented which describe the material exchange at the individual droplets in an extraction with chemical reaction and in liquid membrane permeation.

Bart, H. J.; Bauer, A.; Lorbach, D.; Marr, R.

1988-01-01

289

Non-meanfield deterministic limits in chemical reaction kinetics R. E. Lee DeVille  

E-print Network

Non-meanfield deterministic limits in chemical reaction kinetics R. E. Lee DeVille Courant is demonstrated for the kinetic Monte Carlo version of the Schnakenberg reaction where we identified a scaling, and it is at the very heart of the majority of macroscopic models of physical, chemical, and biological systems

Van Den Eijnden, Eric

290

Motivational Factors Contributing to Turkish High School Students' Achievement in Gases and Chemical Reactions  

ERIC Educational Resources Information Center

This study aimed to investigate the contribution of motivational factors to 10th grade students' achievement in gases and chemical reactions in chemistry. Three hundred fifty nine 10th grade students participated in the study. The Gases and Chemical Reactions Achievement Test and the Motivated Strategies for Learning Questionnaire were…

Kadioglu, Cansel; Uzuntiryaki, Esen

2008-01-01

291

Stochastic simulation of chemical reactions with spatial resolution and single molecule detail  

Microsoft Academic Search

Methods are presented for simulating chemical reaction networks with a spatial resolution that is accurate to nearly the size scale of individual molecules. Using an intuitive picture of chemical reaction systems, each molecule is treated as a point-like particle that diffuses freely in three-dimensional space. When a pair of reactive molecules collide, such as an enzyme and its substrate, a

Steven S. Andrews; Dennis Bray

2004-01-01

292

Introducing Stochastic Simulation of Chemical Reactions Using the Gillespie Algorithm and MATLAB: Revisited and Augmented  

ERIC Educational Resources Information Center

The stochastic simulation of chemical reactions, specifically, a simple reversible chemical reaction obeying the first-order, i.e., linear, rate law, has been presented by Martinez-Urreaga and his collaborators in this journal. The current contribution is intended to complement and augment their work in two aspects. First, the simple reversible…

Argoti, A.; Fan, L. T.; Cruz, J.; Chou, S. T.

2008-01-01

293

Method of operating a thermal engine powered by a chemical reaction  

DOEpatents

The invention involves a novel method of increasing the efficiency of a thermal engine. Heat is generated by a non-linear chemical reaction of reactants, said heat being transferred to a thermal engine such as Rankine cycle power plant. The novel method includes externally perturbing one or more of the thermodynamic variables of said non-linear chemical reaction. 7 figs.

Ross, J.; Escher, C.

1988-06-07

294

Theoretical research program to study chemical reactions in AOTV bow shock tubes  

NASA Technical Reports Server (NTRS)

The main focus was the development, implementation, and calibration of methods for performing molecular electronic structure calculations to high accuracy. These various methods were then applied to a number of chemical reactions and species of interest to NASA, notably in the area of combustion chemistry. Among the development work undertaken was a collaborative effort to develop a program to efficiently predict molecular structures and vibrational frequencies using energy derivatives. Another major development effort involved the design of new atomic basis sets for use in chemical studies: these sets were considerably more accurate than those previously in use. Much effort was also devoted to calibrating methods for computing accurate molecular wave functions, including the first reliable calibrations for realistic molecules using full CI results. A wide variety of application calculations were undertaken. One area of interest was the spectroscopy and thermochemistry of small molecules, including establishing small molecule binding energies to an accuracy rivaling, or even on occasion surpassing, the experiment. Such binding energies are essential input to modeling chemical reaction processes, such as combustion. Studies of large molecules and processes important in both hydrogen and hydrocarbon combustion chemistry were also carried out. Finally, some effort was devoted to the structure and spectroscopy of small metal clusters, with applications to materials science problems.

Taylor, Peter R.

1993-01-01

295

Phase and chemical equilibria in the transesterification reaction of vegetable oils with supercritical lower alcohols  

NASA Astrophysics Data System (ADS)

Calculations of thermodynamic data are performed for fatty acid triglycerides, free fatty acids, and fatty acid methyl esters, participants of the transesterification reaction of vegetable oils that occurs in methanol. Using the obtained thermodynamic parameters, the phase diagrams for the reaction mixture are constructed, and the chemical equilibria of the esterification reaction of free fatty acids and the transesterification reaction of fatty acid triglycerides attained upon treatment with supercritical methanol are determined. Relying on our analysis of the obtained equilibria for the esterification reaction of fatty acids and the transesterification reaction of triglycerides attained upon treatment with lower alcohols, we select the optimum conditions for performing the reaction in practice.

Anikeev, V. I.; Stepanov, D. A.; Ermakova, A.

2011-08-01

296

REGULAR ARTICLE Coordinate reduction for exploring chemical reaction paths  

E-print Network

improve the efficiency of reaction path optimization algorithms. Keywords Reaction path Á PCA Á Potential- ecule as a function of its geometrical parameters [1]. The potential energy surface representing The potential energy surface for the reaction of a typical molecular system composed of N atoms is defined

Schlegel, H. Bernhard

297

Detection of a wide range of medically important fungi by the polymerase chain reaction  

Microsoft Academic Search

Summary. A polymerase chain reaction (PCR) method was developed that was capable of detecting a wide range of medically important fungi from clinical specimens. The primer pair was designed in conserved sequences of 1 8s-ribosomal RNA genes shared by most fungi. The lower limit of detection of this PCR technique was 1 pg of Cundidu ulbicans genomic DNA by ethidium

K. Makimura; SOMAY Y. MURAYAMA; H. Yamaguchi

1994-01-01

298

Molecule-based approach for computing chemical-reaction rates in upper atmosphere hypersonic flows  

Microsoft Academic Search

This report summarizes the work completed during FY2009 for the LDRD project 09-1332 'Molecule-Based Approach for Computing Chemical-Reaction Rates in Upper-Atmosphere Hypersonic Flows'. The goal of this project was to apply a recently proposed approach for the Direct Simulation Monte Carlo (DSMC) method to calculate chemical-reaction rates for high-temperature atmospheric species. The new DSMC model reproduces measured equilibrium reaction rates

Michail A. Gallis; Ryan Bomar Bond; John Robert Torczynski

2009-01-01

299

A comparative study of vibrational relaxation and chemical reaction models for the Martian entry vehicle  

E-print Network

. Chemical Reactions and Rate Coefficients for Set 1 . . 5. Chemical Reactions and Rate Coefficients for Set 2, . 130 . . 131 . . 132 6. Quantum Mechanical Properties of the Species 133 LIST OF FIGURES Figure 1: Typical Mars Entry Vehicles Figure 2.... 9) : Constants in the reaction rate coefficients defined by equations (2. 2) and (2. 3) C?i Cs C, ? CVDV D D; : Molar heat capacity of the i~ species : Translational molar heat capacity of the i~ species : Vibrational molar heat capacity...

Koteshwar, Rajeev

1992-01-01

300

Kinetic models with chemical reactions and nonequilibrium entropy in open systems  

NASA Astrophysics Data System (ADS)

Nonuniform relaxation problems are applied to simulate complex non-equilibrium processes with chemical reactions in open systems. The present paper is a continuation of our studies at a new level, in particular, 2D flows are considered. Besides the slow chemical reactions, the so-called fast reactions are studied. A special attention is paid to studying the behavior of non-equilibrium entropy and entropy flux in such complex open systems.

Aristov, Vladimir; Frolova, Anna; Zabelok, Sergey

2014-12-01

301

Estimation of the Molecular Junction Temperatures in Four-Ball Contacts by Chemical Reaction Rate Studies  

Microsoft Academic Search

A super-refined paraffinic mineral oil has been used to study the chemical reactions under boundary conditions in a four-ball wear tester. By quantatively analyzing the wear debris generated, chemical reaction rate data used to correlate between dynamic wear tests and static, externally controlled temperature runs. The reaction temperature at the wear junction has been estimated to be 351°C for 40kg

Stephen M. Hsu; E. Erwin Klaus

1978-01-01

302

Chemical reaction model for oil and gas generation from type 1 and type 2 kerogen  

SciTech Connect

A global model for the generation of oil and gas from petroleum source rocks is presented. The model consists of 13 chemical species and 10 reactions, including an alternate-pathway mechanism for kerogen pyrolysis. Reaction rate parameters and stoichiometry coefficients determined from a variety of pyrolysis data are given for both type I and type II kerogen. Use of the chemical reaction model is illustrated for typical geologic conditions.

Braun, R.L.; Burnham, A.K.

1993-06-01

303

The effect of reagent energy on chemical reaction rates: An information theoretic analysis  

Microsoft Academic Search

The effect of changing reagent vibrational and rotational energy on the reaction rate has been analyzed for over 20 chemical reactions. In most cases the selectivity in energy requirements could be characterized by a single (''consumption potential'') parameter, even when the reactivity varied by many orders of magnitude. The reactions analyzed covered atom-diatom and diatom-diatom collisions and included both simple

R. D. Levine; J. Manz

1975-01-01

304

A comparison of reversible chemical reactions for solar thermochemical power generation  

E-print Network

453 A comparison of reversible chemical reactions for solar thermochemical power generation O. M storage of the reaction products. A number of reactions have been proposed for solar thermochemical power to be a good choice for first generation solar thermochemical power generation. Revue Phys. Appl. 15 (1980) 453

Boyer, Edmond

305

A Unified Approach to the Study of Chemical Reactions in Freshman Chemistry.  

ERIC Educational Resources Information Center

Provides rationale and objectives for presenting chemical reactions in a unified, logical six-stage approach rather than a piecemeal approach. Stages discussed include: introduction, stable electronic configurations and stable oxidation states, reactions between two free elements, ion transfer/proton transfer reactions, double displacement…

Cassen, T.; DuBois, Thomas D.

1982-01-01

306

CHEMICAL SYNTHESIS USING 'GREENER' ALTERNATIVE REACTION CONDITIONS AND MEDIA  

EPA Science Inventory

The chemical research during the last decade has witnessed a paradigm shift towards "environmentally-friendly chemistry" more popularly known as "green chemistry" due to the increasing environmental concerns and legislative requirements to curb the release of chemical waste into ...

307

SUBSTITUTION REACTIONS FOR THE DETOXIFICATION OF HAZARDOUS CHEMICALS  

EPA Science Inventory

Chemical Treatment is one of several treatment techniques used for the remediation of toxic and hazardous chemicals. Chemical treatment in this report is defined as substitution of halogens by hydrogens for the conversion of halogenated organic toxicant into its native hydrocarb...

308

Vicher: A Virtual Reality Based Educational Module for Chemical Reaction Engineering.  

ERIC Educational Resources Information Center

A virtual reality application for undergraduate chemical kinetics and reactor design education, Vicher (Virtual Chemical Reaction Model) was originally designed to simulate a portion of a modern chemical plant. Vicher now consists of two programs: Vicher I that models catalyst deactivation and Vicher II that models nonisothermal effects in…

Bell, John T.; Fogler, H. Scott

1996-01-01

309

Femtosecond Chemically Activated Reactions: Concept of Nonstatistical Activation at High Thermal Energies  

E-print Network

Energies Sang Kyu Kim, Ju Guo, J. Spencer Baskin, and Ahmed H. Zewail* Arthur Amos Noyes Chemical Physics Femtosecond chemical activation of reactions at very high thermal energies, much above the bond energy relative to chemical binding energies. A central question relevant here is the following: What

Kim, Sang Kyu

310

Multistep One-Pot Reactions Combining Biocatalysts and Chemical Catalysts for Asymmetric Synthesis  

E-print Network

Multistep One-Pot Reactions Combining Biocatalysts and Chemical Catalysts for Asymmetric Synthesis Carl A. Denard, John F. Hartwig,*, and Huimin Zhao*,,§ Department of Chemical and Biomolecular of chemical catalysts. Over the last 20 years, research in this area has provided us with proof of concept

Zhao, Huimin

311

Laboratory studies of Cl and ClO radical reactions of atmospheric importance  

NASA Astrophysics Data System (ADS)

Chlorine released from anthropogenic compounds has been linked to the catalytic destruction of stratospheric ozone. Recently, it has been observed that chlorine, of either anthropogenic or natural origins, initiates oxidation of hydrocarbons in the marine boundary layer. Thus, reactions of chlorine are important for understanding the chemistry of the troposphere and the stratosphere. This study summarizes experimental results obtained using flash photolysis (FP)/tunable diode laser absorption spectroscopy (TDLAS) and discharge flow (DF)/flash photolysis/pulsed laser induced fluorescence (PLIF) techniques. Reaction rate coefficients, yields of products, and distribution of products amongst available vibrational levels were measured. The reactions of Cl atoms with CH3CHO and ClO with OH were studied. The rate coefficient for and the vibrational partitioning of HCl produced in the reaction of Cl atoms with CH3CHO were measured using the FP/TDLAS apparatus. This apparatus was recently designed and assembled. The yields for HCl in this reaction were measured and are in agreement with the previous measurements of its coproducts, CH3CO and CH2CHO. The rate coefficient and branching ratio for the reaction of ClO with OH were measured using the DF/FP/PLIF and FP/TDLAS apparatus, respectively. The coupling of flash photolysis with the discharge flow system has provided a powerful tool in studying radical-radical reactions and in this study, allowed an accurate rate coefficient, k, to be measured. An upper limit for the production of HCl in this reaction was determined. The results obtained are expected to have significant implications in the upper stratosphere.

Kegley-Owen, Carla S.

1998-12-01

312

Mixing and chemical reaction in sheared and nonsheared homogeneous turbulence  

NASA Technical Reports Server (NTRS)

Direct numerical simulations were made to examine the local structure of the reaction zone for a moderately fast reaction between unmixed species in decaying, homogeneous turbulence and in a homogeneous turbulent shear flow. Pseudospectral techniques were used in domains of 64 exp 3 and higher wavenumbers. A finite-rate, single step reaction between non-premixed reactants was considered, and in one case temperature-dependent Arrhenius kinetics was assumed. Locally intense reaction rates that tend to persist throughout the simulations occur in locations where the reactant concentration gradients are large and are amplified by the local rate of strain. The reaction zones are more organized in the case of a uniform mean shear than in isotropic turbulence, and regions of intense reaction rate appear to be associated with vortex structures such as horseshoe vortices and fingers seen in mixing layers. Concentration gradients tend to align with the direction of the most compressive principal strain rate, more so in the isotropic case.

Leonard, Andy D.; Hill, James C.

1992-01-01

313

Laboratory simulations of chemical reactions on dust grains in the interstellar medium  

NASA Astrophysics Data System (ADS)

Dust grains exert a major influence upon the chemical composition of the interstellar medium: photoelectrons emitted from the dust grains are the primary energy source for heating interstellar gas, dust grains in dense molecular clouds can accumulate layers of frozen interstellar gases that participate in solid phase chemical reactions, and the most abundant molecule in the Universe, molecular hydrogen, primarily forms from hydrogen atoms adsorbed onto grain surfaces. Molecular hydrogen influences the evolution of molecular clouds by acting as a coolant during the gravitational collapse of the cloud and serving as a precursor for the formation of many molecular species. A complete description of molecular hydrogen formation in molecular clouds requires an understanding of the efficiency of hydrogen atom recombination on ice surfaces. Observations of interstellar carbon dioxide ice have the potential for serving as a diagnostic sign of the evolution of interstellar ice layers but require a satisfactory explanation of the formation mechanisms of interstellar CO 2 . This work describes a series of investigations that were designed to study the properties of interstellar dust grains and to obtain and analyze data for astrophysically important chemical reactions. We measured the recombination efficiency of H atoms on the surface of amorphous H 2 O ices and measured the kinetics of H 2 formation and desorption on different morphologies of ice substrate. We demonstrated that the hydrogen atom recombination kinetics depend upon the morphology of the ice layer and that the recombination efficiency is consistent with observations of molecular clouds. We also demonstrated that CO and O can be trapped within an amorphous H 2 O ice layer at temperatures greater than their sublimation temperatures and that the reaction CO (ads) + O (ads) [arrow right] CO 2,(ads) can produce appreciable amounts of CO2 within an interstellar ice layer in the absence of ultraviolet or cosmic-ray irradiation of the ice mantles. Extensions of these experimental techniques for future investigations of chemistry on the surfaces of interstellar dust grain analogues are also discussed.

Roser, Joseph E.

314

A lattice gas automata model for heterogeneous chemical reactions at mineral surfaces and in pore networks  

SciTech Connect

A lattice gas automata (LGA) model is described, which couples solute transport with chemical reactions at mineral surfaces and in pore networks. Chemical reactions and transport are integrated into a FHP-I LGA code as a module so that the approach is readily transportable to other codes. Diffusion in a box calculations are compared to finite element Fickian diffusion results and provide an approach to quantifying space-time ratios of the models. Chemical reactions at solid surfaces, including precipitation/dissolution, sorption, and catalytic reaction, can be examined with the model because solute diffusion and mineral surface processes are all treated explicitly. The simplicity and flexibility of the LGA approach provides the ability to study the interrelationship between fluid flow and chemical reactions in porous materials, at a level of complexity that has not previously been computationally possible. 20 refs., 8 figs.

Wells, J.T. (Washington Univ., Seattle, WA (USA). Dept. of Geological Sciences); Janecky, D.R.; Travis, B.J. (Los Alamos National Lab., NM (USA))

1990-01-15

315

Characterization of chemically modified hyperthermophilic enzymes for chemical syntheses and bioremediation reactions. 1998 annual progress report  

SciTech Connect

'Remediation processes frequently involve species possessing limited solubility in water. The authors are interested in novel strategies that use molecularly-engineered enzymes with enhanced activity and stability for the remediation of recalcitrant compounds in organic media. Organic biocatalysis may be motivated by the nature of the substrate itself, or by augmented mass transport, ease of product recovery, or novel reaction pathways afforded by the organic solvent. However, naturally-occurring enzymes are usually subject to quite limited activity and stability in such organic environments. The objective of the current work is to gain a fundamental understanding of the molecular and catalytic properties of enzymes that have been chemically-modified so that they are catalytically-active and chemically-thermally-stable in organic solvents. The premise for this study is that highly stable enzymes which are catalytically active in both water and in a range of organic solvents are optimally suited for bioremediation where substrates of interest are more soluble and may be processed with greater specificity in nonaqueous solvents. This bioprocessing system will be assessed using PCB''s, DNAPL''s and PAH''s. As of the second of year of a 3-year project, the authors have obtained promising results for both the demonstration of the proposed remediation strategy and fundamental understanding of the enzymatic catalysis in organic media. In one of the efforts, ligninase (LiP) was modified by poly(ethylene glycol) (PEG) and examined for the degradation of pentachlorophenol (PCP) in water-solvent mixtures. In the other efforts, basic catalytic behaviors of the chemically modified hyperthermophilic metalloenzymes including ferredoxin, hydrogenase and aldehyde oxidoreductase were examined in organic solvents.'

Kaufman, E.N. [Oak Ridge National Lab., TN (US); Adams, M.W.W. [Univ. of Georgia, Athens, GA (US)

1998-06-01

316

Molecule-based approach for computing chemical-reaction rates in upper atmosphere hypersonic flows.  

SciTech Connect

This report summarizes the work completed during FY2009 for the LDRD project 09-1332 'Molecule-Based Approach for Computing Chemical-Reaction Rates in Upper-Atmosphere Hypersonic Flows'. The goal of this project was to apply a recently proposed approach for the Direct Simulation Monte Carlo (DSMC) method to calculate chemical-reaction rates for high-temperature atmospheric species. The new DSMC model reproduces measured equilibrium reaction rates without using any macroscopic reaction-rate information. Since it uses only molecular properties, the new model is inherently able to predict reaction rates for arbitrary nonequilibrium conditions. DSMC non-equilibrium reaction rates are compared to Park's phenomenological non-equilibrium reaction-rate model, the predominant model for hypersonic-flow-field calculations. For near-equilibrium conditions, Park's model is in good agreement with the DSMC-calculated reaction rates. For far-from-equilibrium conditions, corresponding to a typical shock layer, the difference between the two models can exceed 10 orders of magnitude. The DSMC predictions are also found to be in very good agreement with measured and calculated non-equilibrium reaction rates. Extensions of the model to reactions typically found in combustion flows and ionizing reactions are also found to be in very good agreement with available measurements, offering strong evidence that this is a viable and reliable technique to predict chemical reaction rates.

Gallis, Michail A.; Bond, Ryan Bomar; Torczynski, John Robert

2009-08-01

317

CHEMICAL REACTIONS OF AQUATIC HUMIC MATERIALS WITH SELECTED OXIDANTS  

EPA Science Inventory

A study was conducted to identify the specific organic reaction products of natural aquatic humic materials with selected oxidants (KMnO4, HOCl, Cl02, O3 and monochloramine). Reaction products were identified by GC/MS after solvent extraction and derivatization. The two most reac...

318

Classification of chemical substances, reactions, and interactions: The effect of expertise  

NASA Astrophysics Data System (ADS)

This project explored the strategies that undergraduate and graduate chemistry students engaged in when solving classification tasks involving microscopic (particulate) representations of chemical substances and microscopic and symbolic representations of different chemical reactions. We were specifically interested in characterizing the basic features to which students pay attention while classifying, identifying the patterns of reasoning that they follow, and comparing the performance of students with different levels of preparation in the discipline. In general, our results suggest that advanced levels of expertise in chemical classification do not necessarily evolve in a linear and continuous way with academic training. Novice students had a tendency to reduce the cognitive demand of the task and rely on common-sense reasoning; they had difficulties differentiating concepts (conceptual undifferentiation) and based their classification decisions on only one variable (reduction). These ways of thinking lead them to consider extraneous features, pay more attention to explicit or surface features than implicit features and to overlook important and relevant features. However, unfamiliar levels of representations (microscopic level) seemed to trigger deeper and more meaningful thinking processes. On the other hand, expert students classified entities using a specific set of rules that they applied throughout the classification tasks. They considered a larger variety of implicit features and the unfamiliarity with the microscopic level of representation did not affect their reasoning processes. Consequently, novices created numerous small groups, few of them being chemically meaningful, while experts created few but large chemically meaningful groups. Novices also had difficulties correctly classifying entities in chemically meaningful groups. Finally, expert chemists in our study used classification schemes that are not necessarily traditionally taught in classroom chemistry (e.g. the structure of substances is more relevant to them than their composition when classifying substances as compounds or elements). This result suggests that practice in the field may develop different types of knowledge framework than those usually presented in chemistry textbooks.

Stains, Marilyne Nicole Olivia

2007-12-01

319

Soret and chemical reaction effects on unsteady two-dimensional natural convection along a vertical plate  

NASA Astrophysics Data System (ADS)

In this paper, a numerical solution of the unsteady two-dimensional natural convection along a vertical plate in the presence of Soret and chemical reaction effects is presented. The governing non-dimensional coupled non-linear partial differential equations have been evaluated by using an implicit finite-difference technique of Crank-Nicolson scheme. Numerical predictions for the velocity, concentration, local and average skin-friction and Sherwood number for distinct values of chemical reaction parameter and Soret number are plotted graphically. It is found that the fluid velocity and concentration decreases while increasing chemical reaction parameter whereas an increase in the Soret number increases the fluid velocity and concentration.

Raju, S. Suresh Kumar; Narahari, M.; Pendyala, Rajashekhar

2014-10-01

320

NEURAL - a tracking detector for neutron-induced reactions of astrophysical importance  

NASA Astrophysics Data System (ADS)

Observations from gamma ray telescopes indicate that most of the galactic 26Al originates in massive stars. Several sites have been discussed for its production, including proton burning in the winds of very massive stars, and the later, explosive burning stages of these stars. Observations from the RHESSI and INTEGRAL missions currently seem to point to the latter scenario. In the advanced burning stages of massive stars the presence of neutrons becomes an important factor in nuclear reaction networks, so in addition to the 26Al(p,?)27Si reaction the neutron capture reactions 26Al(n,p)26Mg and 26Al(n,?)23Na can lead to the destruction of 26Al, and thus alter the observed 26Al abundance. NEURAL is a detector design to measure the excitation functions of these reactions over a wide range of energies. 26Al targets implanted at TRIUMF will be exposed to a pulsed neutron beam at the neutron time-of-flight facility at LANSCE (Los Alamos Neutron Science CEnter). NEURAL is designed to detect all charged reaction products, combining a time projection chamber for the heavy ions, and Si detectors for the light particles mounted around the target. A first prototype has been built and partially tested at TRIUMF and LANSCE in December 2009.

Martin, L.; Buchmann, L.; Carpino, J. F.; Chen, A.; Couture, A.; Davids, B.; Fallis, J.; Fox, S. P.; Hager, U.; Hussein, A.; Laird, A. M.; Olchanski, K.; Ottewell, D.; Ruiz, C.; Ruprecht, G.; Sheffer, G.; Sjue, S.; Tengblad, O.; Tovesson, F.

2011-09-01

321

Tattoo-Associated Skin Reaction: The Importance of an Early Diagnosis and Proper Treatment  

PubMed Central

Tattoo is going to be a very common practice especially among young people and we are witnessing a gradual increase of numerous potential complications to tattoo placement which are often seen by physicians, but generally unknown to the public. The most common skin reactions to tattoo include a transient acute inflammatory reaction due to trauma of the skin with needles and medical complications such as superficial and deep local infections, systemic infections, allergic contact dermatitis, photodermatitis, granulomatous and lichenoid reactions, and skin diseases localized on tattooed area (eczema, psoriasis, lichen, and morphea). Next to these inflammatory skin reactions we have to consider also the possibility of the development of cutaneous conditions such as pseudolymphomatous reactions and pseudoepitheliomatous hyperplasia. The aim of this study is to underline the importance of an early diagnosis by performing a histological examination especially when we are in front of suspected papulonodular lesions arising from a tattoo, followed by a proper treatment, since cutaneous neoplastic evolution is known to be a rare but possible complication. PMID:25147796

Bassi, Andrea; Campolmi, Piero; Cannarozzo, Giovanni; Conti, Rossana; Bruscino, Nicola; Gola, Massimo; Ermini, Stefano; Massi, Daniela; Moretti, Silvia

2014-01-01

322

Students' perceptions, attitudes, and incorporation of demonstrations, popular media videos, and animations concerning chemical reactions  

NASA Astrophysics Data System (ADS)

Students often struggle with learning complex chemistry concepts. In today's society with the advances in multimedia technology, educators have a variety of tools available to help students learn these concepts. These tools include demonstrations, videos in the popular media, and animations; referred to collectively as multimethods. With the increase in possibilities educators must consider a variety of factors, the most important of which is their students. An examination of the literature leaves questions regarding the views of students as well as the impact of these tools on student learning. This study qualitatively explores student perceptions and attitudes regarding each of these multimethods using open-ended surveys, worksheets, interviews, and audio recordings. Concept maps and drawings provide insight into how students incorporate information from these multimethods into their understanding of chemical reactions. The results of the study emphasize variety for student perceptions, attitudes, conceptions, and the way they incorporate information from the multimethods. However, there also were trends in that the majority of students preferred demonstrations and the macro level. The students appeared to be aware that the use of these multimethods is designed to help them learn. Student drawings and concept maps additionally indicate a lack of understanding of the nature of chemical reactions and a failure to incorporate the information in a substantial way. Finally, implications for teaching are discussed.

Schlosser, Sarah Elizabeth

323

RPMDrate: Bimolecular chemical reaction rates from ring polymer molecular dynamics  

E-print Network

We present RPMDrate, a computer program for the calculation of gas phase bimolecular reaction rate coefficients using the ring polymer molecular dynamics (RPMD) method. The RPMD rate coefficient is calculated using the ...

Suleimanov, Yu.V.

324

Efficiency of a stirred chemical reaction in a closed vessel  

E-print Network

We perform a numerical study of the reaction efficiency in a closed vessel. Starting with a little spot of product, we compute the time needed to complete the reaction in the container following an advection-reaction-diffusion process. Inside the vessel it is present a cellular velocity field that transports the reactants. If the size of the container is not very large compared with the typical length of the velocity field one has a plateau of the reaction time as a function of the strength of the velocity field, $U$. This plateau appears both in the stationary and in the time-dependent flow. A comparison of the results for the finite system with the infinite case (for which the front speed, $v_f$, gives a simple estimate of the reacting time) shows the dramatic effect of the finite size.

Cristobal lopez; Davide Vergni; Angelo Vulpiani

2002-01-22

325

Role of Conformational Structures and Torsional Anharmonicity in Controlling Chemical Reaction Rates and Relative Yields: Butanal + HO2 Reactions  

SciTech Connect

Aldehyde–radical reactions are important in atmospheric and combustion chemistry, and the reactions studied here also serve more generally to illustrate a fundamental aspect of chemical kinetics that has been relatively unexplored from a quantitative point of view, in particular the roles of multiple structures and torsional anharmonicity in determining the rate constants and branching ratios (product yields). We consider hydrogen abstraction from four carbon sites of butanal (carbonyl-C, a-C, b-C and g-C) by hydroperoxyl radical. We employed multi-structural variational transition state theory for studying the first three channels; this uses a multi-faceted dividing surface and allows us to include the contributions of multiple structures of both reacting species and transition states. Multiconfigurational Shepard interpolation (MCSI) was used to obtain the geometries and energies of the potential energy surface along the minimum-energy paths, with gradients and Hessians calculated by the M08-HX/maug-cc-pVTZ method. We find the numbers of structures obtained for the transition states are 46, 60, 72 and 76respectively for the H abstraction at the carbonyl C, the a position, the b position and the g position. Our results show that neglecting the factors arising from multiple structures and torsional anharmonicity would lead to errors at 300, 1000 and 2400 K of factors of 8, 11 and 10 for abstraction at the carbonyl-O, 2, 11 and 25 at the a-C position, 2, 23 and 47 at the b-C position, and 0.6, 8 and 18 at the g-C position. The errors would be even larger at high temperature for the reverse of the H abstraction at the b-C. Relative yields are changed as much as a factor of 7.0 at 200 K, a factor of 5.0 at 298 K, and a factor of 3.7 in the other direction at 2400 K. The strong dependence of the product ratios on the multi-structural anharmonicity factors shows that such factors play an important role in controlling branching ratios in reaction mechanism networks.

Zheng, Jingjing; Seal, Prasenjit; Truhlar, Donald G.

2013-01-01

326

reactions  

Microsoft Academic Search

Gene regulatory, signal transduction and metabolic networks are major areas of interest in the newly emerging field of systems biology. In living cells, stochastic dynamics play an important role; however, the kinetic parameters of biochemical reactions necessary for modelling these processes are often not accessible directly through experiments. The problem of estimating stochastic reaction constants from molecule count data measured,

S. Reinker; R. M. Altman; J. Timmer

327

Conceptual Design of Biorefineries Through the Synthesis of Optimal Chemical-reaction Pathways  

E-print Network

on developing new pathways while optimizing existing ones. Here, potential chemicals are added to create a superstructure, then an algorithm is run to enumerate every feasible reaction stoichiometry through a mixed integer linear program (MILP). An optimal...

Pennaz, Eric James

2011-10-21

328

Tungsten-dependent formaldehyde ferredoxin oxidoreductase: Reaction mechanism from quantum chemical calculations  

E-print Network

Tungsten-dependent formaldehyde ferredoxin oxidoreductase: Reaction mechanism from quantum chemical theory Enzyme catalysis Formaldehyde ferredoxin oxidoreductase from Pyrococcus furiosus is a tungsten the formaldehyde substrate binds directly to the tungsten ion. WVI =O then performs a nucleophilic attack

Liao, Rongzhen

329

Descriptive Simulation: Combining Symbolic and Numerical Methods in the Analysis of Chemical Reaction Mechanisms  

E-print Network

The Kineticist's Workbench is a computer program currently under development whose purpose is to help chemists understand, analyze, and simplify complex chemical reaction mechanisms. This paper discusses one module ...

Eisenberg, Michael

1989-09-01

330

The Kineticist's Workbench: Combining Symbolic and Numerical Methods in the Simulation of Chemical Reaction Mechanisms  

E-print Network

The Kineticist's Workbench is a program that simulates chemical reaction mechanisms by predicting, generating, and interpreting numerical data. Prior to simulation, it analyzes a given mechanism to predict that ...

Eisenberg, Michael A.

1991-05-01

331

EVALUATION OF CHEMICAL REACTION MECHANISMS FOR PHOTOCHEMICAL SMOG. PART 2. QUANTITATIVE EVALUATION OF THE MECHANISMS (REVISED)  

EPA Science Inventory

Six chemical reaction mechanisms for photochemical smog were analyzed to determine why, under identical conditions, they predict different maximum ozone concentrations. To perform the analysis, a counter species technique was used to determine the contributions of individual reac...

332

LSENS, a general chemical kinetics and sensitivity analysis code for gas-phase reactions: User's guide  

NASA Technical Reports Server (NTRS)

A general chemical kinetics and sensitivity analysis code for complex, homogeneous, gas-phase reactions is described. The main features of the code, LSENS, are its flexibility, efficiency and convenience in treating many different chemical reaction models. The models include static system, steady, one-dimensional, inviscid flow, shock initiated reaction, and a perfectly stirred reactor. In addition, equilibrium computations can be performed for several assigned states. An implicit numerical integration method, which works efficiently for the extremes of very fast and very slow reaction, is used for solving the 'stiff' differential equation systems that arise in chemical kinetics. For static reactions, sensitivity coefficients of all dependent variables and their temporal derivatives with respect to the initial values of dependent variables and/or the rate coefficient parameters can be computed. This paper presents descriptions of the code and its usage, and includes several illustrative example problems.

Radhakrishnan, Krishnan; Bittker, David A.

1993-01-01

333

Large-scale prediction of adverse drug reactions using chemical, biological, and phenotypic properties of drugs  

PubMed Central

Objective Adverse drug reaction (ADR) is one of the major causes of failure in drug development. Severe ADRs that go undetected until the post-marketing phase of a drug often lead to patient morbidity. Accurate prediction of potential ADRs is required in the entire life cycle of a drug, including early stages of drug design, different phases of clinical trials, and post-marketing surveillance. Methods Many studies have utilized either chemical structures or molecular pathways of the drugs to predict ADRs. Here, the authors propose a machine-learning-based approach for ADR prediction by integrating the phenotypic characteristics of a drug, including indications and other known ADRs, with the drug's chemical structures and biological properties, including protein targets and pathway information. A large-scale study was conducted to predict 1385 known ADRs of 832 approved drugs, and five machine-learning algorithms for this task were compared. Results This evaluation, based on a fivefold cross-validation, showed that the support vector machine algorithm outperformed the others. Of the three types of information, phenotypic data were the most informative for ADR prediction. When biological and phenotypic features were added to the baseline chemical information, the ADR prediction model achieved significant improvements in area under the curve (from 0.9054 to 0.9524), precision (from 43.37% to 66.17%), and recall (from 49.25% to 63.06%). Most importantly, the proposed model successfully predicted the ADRs associated with withdrawal of rofecoxib and cerivastatin. Conclusion The results suggest that phenotypic information on drugs is valuable for ADR prediction. Moreover, they demonstrate that different models that combine chemical, biological, or phenotypic information can be built from approved drugs, and they have the potential to detect clinically important ADRs in both preclinical and post-marketing phases. PMID:22718037

Liu, Mei; Wu, Yonghui; Chen, Yukun; Sun, Jingchun; Zhao, Zhongming; Chen, Xue-wen; Matheny, Michael Edwin

2012-01-01

334

Theoretical study of thermodynamic properties and reaction rates of importance in the high-speed research program  

NASA Technical Reports Server (NTRS)

One of the primary goals of NASA's high-speed research program is to determine the feasibility of designing an environmentally safe commercial supersonic transport airplane. The largest environmental concern is focused on the amount of ozone destroying nitrogen oxides (NO(x)) that would be injected into the lower stratosphere during the cruise portion of the flight. The limitations placed on NO(x) emission require more than an order of magnitude reduction over current engine designs. To develop strategies to meet this goal requires first gaining a fundamental understanding of the combustion chemistry. To accurately model the combustor requires a computational fluid dynamics approach that includes both turbulence and chemistry. Since many of the important chemical processes in this regime involve highly reactive radicals, an experimental determination of the required thermodynamic data and rate constants is often very difficult. Unlike experimental approaches, theoretical methods are as applicable to highly reactive species as stable ones. Also our approximation of treating the dynamics classically becomes more accurate with increasing temperature. In this article we review recent progress in generating thermodynamic properties and rate constants that are required to understand NO(x) formation in the combustion process. We also describe our one-dimensional modeling efforts to validate an NH3 combustion reaction mechanism. We have been working in collaboration with researchers at LeRC, to ensure that our theoretical work is focused on the most important thermodynamic quantities and rate constants required in the chemical data base.

Langhoff, Stephen; Bauschlicher, Charles; Jaffe, Richard

1992-01-01

335

High-temperature in situ magic-angle spinning NMR studies of chemical reactions on catalysts  

E-print Network

HIGH-TEMPERATURE IN SITU MAGIC-ANGLE SPINNING NMR STUDIES OF CHEMICAL REACTIONS ON CATALYSTS by F. GREGORY OLIVER Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements... for the degree MASTER OF SCIENCE August 1992 Major Sublect: Chemistry HIGH-TEMPERATURE IN SITU MAGIC-ANGLE SPINNING NMR STUDIES OF CHEMICAL REACTIONS ON CATALYSTS A Thesis by F. GREGORY OLIVER Approved as to style and content by: James F. Haw (Chair...

Oliver, F. Gregory

1992-01-01

336

Transient modeling of chemical vapor infiltration of methane using multi-step reaction and deposition models  

Microsoft Academic Search

Based on multi-step reaction and deposition models including the hydrogen inhibition model of pyrocarbon growth, transient 2D simulations of chemical vapor infiltration of methane were carried out by a finite element method (FEM) coupling the mass transfer (by convection and diffusion) and the evolutive surface area model with gas-phase and surface chemical reactions. The continuous infiltration, pyrolysis and deposition of

Aijun Li; Olaf Deutschmann

2007-01-01

337

15 CFR 713.3 - Annual declaration and reporting requirements for exports and imports of Schedule 2 chemicals.  

Code of Federal Regulations, 2011 CFR

...for exports and imports of Schedule 2 chemicals. 713.3 Section 713.3 Commerce...SECURITY, DEPARTMENT OF COMMERCE CHEMICAL WEAPONS CONVENTION REGULATIONS ACTIVITIES INVOLVING SCHEDULE 2 CHEMICALS § 713.3 Annual...

2011-01-01

338

15 CFR 713.3 - Annual declaration and reporting requirements for exports and imports of Schedule 2 chemicals.  

Code of Federal Regulations, 2010 CFR

...for exports and imports of Schedule 2 chemicals. 713.3 Section 713.3 Commerce...SECURITY, DEPARTMENT OF COMMERCE CHEMICAL WEAPONS CONVENTION REGULATIONS ACTIVITIES INVOLVING SCHEDULE 2 CHEMICALS § 713.3 Annual...

2010-01-01

339

Kinetics of thermochemical gas-solid reactions important in the Venus sulfur cycle  

NASA Technical Reports Server (NTRS)

The thermochemical net reaction CaCO3 + SO2 yields CaSO4 + CO is predicted to be an important sink for incorporation of SO2 into the Venus crust. The reaction rate law was established to understand the dependence of rate on experimental variables such as temperature and partial pressure of SO2, CO2, and O2. The experimental approach was a variant of the thermogravimetric method often employed to study the kinetics of thermochemical gas-solid reactions. Clear calcite crystals were heated at constant temperature in SO2-bearing gas streams for varying time periods. Reaction rate was determined by three independent methods. A weighted linear least squares fit to all rate data yielded a rate equation. Based on the Venera 13, 14 and Vega 2 observations of CaO content of the Venus atmosphere, SO2 at the calculated rate would be removed from the Venus atmosphere in about 1,900,00 years. The most plausible endogenic source of the sulfur needed to replenish atmospheric SO2 is volcanism. The annual amount of erupted material needed for the replenishment depends on sulfur content; three ratios are used to calculate rates ranging from 0.4 to 11 cu km/year. This geochemically derived volcanism rate can be used to test if geophysically derived rates are correct. The work also suggests that Venus is less volcanically active than the Earth.

Fegley, Bruce, Jr.

1988-01-01

340

Ozonolysis of Mixed Oleic-Acid/Stearic-Acid Particles: Reaction Kinetics and Chemical Morphology  

NASA Astrophysics Data System (ADS)

Atmospheric particles directly and indirectly affect global climate and have a primary role in regional issues of air pollution, visibility, and human health. Atmospheric particles have a variety of shapes, dimensions, and chemical compositions, and these physicochemical properties evolve (i.e., "age") during transport of the particles through the atmosphere, in part because of the chemical reactions of particle-phase organic molecules with gas-phase atmospheric oxidants. As a global average, hydroxyl radical (OH) and ozone (O3) are responsible quantitatively for most oxidant aging of atmospheric particles. The reactions of the hydroxyl radical occur in the surface region of a particle because of the nearly diffusion-limited bimolecular rate constant of OH with a variety of organic molecules. Ozone, on the other hand, is a selective agent for the unsaturated bonds of organic molecules and may diffuse a considerable distance into particles prior to reaction. The reaction of oleic acid with ozone has recently emerged as a model system to better understand the atmospheric chemical oxidation processes affecting organic particles. The ozonolysis of mixed oleic-acid/stearic-acid (OL/SA) aerosol particles from 0/100 to 100/0 weight percent composition is studied. The magnitude of the divergence of the particle beam inside an aerosol mass spectrometer shows that, in the concentration range 100/0 to 60/40, the mixed OL/SA particles are liquid prior to reaction. Upon ozonolysis, particles with SA composition greater than 25% change shape, indicating that they have solidified. Transmission electron micrographs show that SA(s) forms needles. For SA compositions greater than 10%, the reaction kinetics exhibit an initial fast decay of OL for low O3 exposure with no further loss of OL at higher O3 exposures. For compositions from 50/50 to 10/90, the residual OL concentration remains at 28+/-2% of its initial value. The initial reactive uptake coefficient for O3, as determined by OL loss, decreases linearly from 1.25(+/-0.2) 10-3 to 0.60(+/-0.15) 10-3 for composition changes of 100/0 to 60/40. At 50/50 composition, the uptake coefficient drops abruptly to 0.15(+/-0.1) 10-3, and there are no further changes with increased SA content. The amount of SA in the particles also decreases during OL ozonolysis. The stabilized Criegee intermediate (SCI) formed by OL ozonolysis attacks the carboxylic acid group of SA to yield an acyloxyalkyl hydroperoxide product. The experimental observations of this study can be explained by the following two postulates: (1) unreacted mixed particles remain as supersatured liquids up to 60/40 composition and (2) SA, as it solidifies, locks in a significant amount of oleic acid. The results of this study point out the important effects of particle phase, composition, and morphology on chemical reactivity. Oleic acid in liquid regions of a particle reacts rapidly with O3 whereas OL trapped inside solid SA is essentially unavailable for reaction with O3. These results contribute to the continuing development of the scientific community's understanding of particle aging process in the atmosphere, for which the ultimate goal is to provide quantitative mechanistic models of physicochemical transformations under atmospheric conditions.

Martin, S. T.; Katrib, Y.; Biskos, G.; Buseck, P. R.; Davidovits, P.; Jayne, J. T.; Mochida, M.; Wise, M. E.; Worsnop, D. R.

2005-12-01

341

Relationship between Thermodynamic Driving Force and One-Way Fluxes in Reversible Chemical Reactions  

E-print Network

Chemical reaction systems operating in nonequilibrium open-system states arise in a great number of contexts, including the study of living organisms, in which chemical reactions, in general, are far from equilibrium. Here we introduce a theorem that relates forward and re-verse fluxes and free energy for any chemical process operating in a steady state. This rela-tionship, which is a generalization of equilibrium conditions to the case of a chemical process occurring in a nonequilibrium steady state, provides a novel equivalent definition for chemical reaction free energy. In addition, it is shown that previously unrelated theories introduced by Ussing and Hodgkin and Huxley for transport of ions across membranes, Hill for catalytic cycle fluxes, and Crooks for entropy production in microscopically reversible systems, are united in a common framework based on this relationship.

Daniel A. Beard; Hong Qian

2006-11-22

342

Light-induced nitrous acid (HONO) production from NO2 heterogeneous reactions on household chemicals  

NASA Astrophysics Data System (ADS)

Nitrous acid (HONO) can be generated in various indoor environments directly during combustion processes or indirectly via heterogeneous NO2 reactions with water adsorbed layers on diverse surfaces. Indoors not only the concentrations of NO2 are higher but the surface to volume (S/V) ratios are larger and therefore the potential of HONO production is significantly elevated compared to outdoors. It has been claimed that the UV solar light is largely attenuated indoors. Here, we show that solar light (? > 340 nm) penetrates indoors and can influence the heterogeneous reactions of gas-phase NO2 with various household surfaces. The NO2 to HONO conversion mediated by light on surfaces covered with domestic chemicals has been determined at atmospherically relevant conditions i.e. 50 ppb NO2 and 50% RH. The formation rates of HONO were enhanced in presence of light for all the studied surfaces and are determined in the following order: 1.3·109 molecules cm-2 s-1 for borosilicate glass, 1.7·109 molecules cm-2 s-1 for bathroom cleaner, 1.0·1010 molecules cm-2 s-1 on alkaline detergent (floor cleaner), 1.3·1010 molecules cm-2 s-1 for white wall paint and 2.7·1010 molecules cm-2 s-1 for lacquer. These results highlight the potential of household chemicals, used for cleaning purposes to generate HONO indoors through light-enhanced NO2 heterogeneous reactions. The results obtained have been applied to predict the timely evolution of HONO in a real indoor environment using a dynamic mass balance model. A steady state mixing ratio of HONO has been estimated at 1.6 ppb assuming a contribution from glass, paint and lacquer and considering the photolysis of HONO as the most important loss process.

Gómez Alvarez, Elena; Sörgel, Matthias; Gligorovski, Sasho; Bassil, Sabina; Bartolomei, Vincent; Coulomb, Bruno; Zetzsch, Cornelius; Wortham, Henri

2014-10-01

343

Influence of mineral weathering reactions on the chemical composition of soil water, springs, and ground water, Catoctin Mountains, Maryland  

USGS Publications Warehouse

During 1983 and 1984, wet precipitation was primarily a solution of dilute sulphuric acid, whereas calcium and bicarbonate were the major ions in springs and ground water in two small watersheds with a deciduous forest cover in central Maryland. Dominant ions in soil water were calcium, magnesium, and sulphate. The relative importance of mineral weathering reactions on the chemical composition of these subsurface waters was compared to the contribution from wet precipitation, biological processes, and road deicing salts. -from Author

Katz, B.G.

1989-01-01

344

Analytical Solution of Steady State Equations for Chemical Reaction Networks with Bilinear Rate Laws  

PubMed Central

True steady states are a rare occurrence in living organisms, yet their knowledge is essential for quasi-steady state approximations, multistability analysis, and other important tools in the investigation of chemical reaction networks (CRN) used to describe molecular processes on the cellular level. Here we present an approach that can provide closed form steady-state solutions to complex systems, resulting from CRN with binary reactions and mass-action rate laws. We map the nonlinear algebraic problem of finding steady states onto a linear problem in a higher dimensional space. We show that the linearized version of the steady state equations obeys the linear conservation laws of the original CRN. We identify two classes of problems for which complete, minimally parameterized solutions may be obtained using only the machinery of linear systems and a judicious choice of the variables used as free parameters. We exemplify our method, providing explicit formulae, on CRN describing signal initiation of two important types of RTK receptor-ligand systems, VEGF and EGF-ErbB1. PMID:24334389

Halász, Ádám M.; Lai, Hong-Jian; McCabe, Meghan M.; Radhakrishnan, Krishnan; Edwards, Jeremy S.

2014-01-01

345

Reaction operators for spin-selective chemical reactions of radical pairs  

E-print Network

Spin-selective reactions of radical pairs have traditionally been modelled theoretically by adding phenomenological rate equations to the quantum mechanical equation of motion of the radical pair spin density matrix. More recently an alternative set of rate expressions, based on a quantum measurement approach, has been suggested. Here we show how these two reaction operators can be seen as limiting cases of a more general reaction scheme.

Jonathan A. Jones; Kiminori Maeda; Peter J. Hore

2011-03-30

346

The Importance of the Entropy Inequality on Numerical Simulations Using Reduced Methane-air Reaction Mechanisms  

E-print Network

mechanisms, Mech 1 and Mech 2, are developed from the original two-step reaction mechanism by modifying the empirical data constants in the Arrhenius reaction form. The reaction exponents are set to the stoichiometric coefficients of the reaction...

Jones, Nathan

2012-10-19

347

Chemical reaction and equilibration mechanisms in detonation waves  

SciTech Connect

Experimental and theoretical evidence for the nonequilibrium Zeldovich-von Neumann-Doring (NEZND) theory of self-sustaining detonation is presented. High density, high temperature transition state theory is used to calculate unimolecular reaction rate constants for the initial decomposition of gaseous norbornene, liquid nitromethane, and solid, single crystal pentaerythritol tetranitrate as functions of shock temperature. The calculated rate constants are compared to those derived from experimental induction time measurements at various shock and detonation states. Uncertainties in the calculated shock and von Neumann spike temperatures are the main drawbacks to calculating these reaction rates. Nanosecond measurements of the shock temperatures of unreacted explosives are necessary to reduce these uncertainties.

Tarver, C. M., LLNL

1997-07-01

348

Production of a Gas- Controlling a Chemical Reaction  

NSDL National Science Digital Library

In this activity, students have seen a few reactions that produce a gas.They will adjust the amount of baking soda and vinegar to control the amount of gas produced in the reaction. The materials needed are basic and easily available anywhere. There is a downloadable activity sheet that will be very helpful to educators, and will help students stay on track. An assessment sheet is also available on the activity page to keep track of students progress. There is also a step by step guide as to how to perform the experiment, and how to introduce it t the students.

James Kessler

2010-01-01

349

Energy storage and transport by reversible chemical reactions  

NASA Astrophysics Data System (ADS)

Reversible thermochemical reactions are one of the possibilities to store and transport high temperature heat (800 K up to 1300 K). There are open cycles and closed cycles. Some reversible systems are described, as the SO2-SO3 system. A typical example of open cycle is the reaction for water decomposition. Results of a pilot plant to verify the decomposition of sulfuric acid are described; the technological feasibility of this method for hydrogen production is shown. The possibility to use other high temperature sources, as solar energy is discussed.

Beghi, G.

350

Developing a New Air Pollution Dispersion Model with Chemical Reactions Based on Multiple Cell Approach  

Microsoft Academic Search

A two dimensional atmospheric dispersion model for computation of the ambient air concentration of reactive pollutants emitted from ground level sources is described. Atmospheric chemical reactions are the most complicated and stiff part of pollutants dispersion equations. Coupling them with other physical transport processes to assemble an integrated dispersion model is a time consuming and complicated matter. Mechanism of reactions

Ali Alizadeh Osalu; Mohammad Ali Kaynejad; Esmaeil Fatehifar; ALI ELKAMEL

2009-01-01

351

Magnetic-field dependence of chemical reaction rates at high temperatures  

Microsoft Academic Search

The theory of magnetic-field dependence of chemical reaction rates at high temperatures is discussed. It is shown that in the zero-order approximation in the parameter (muH\\/kT)2 this dependence can be significant. It depends crucially on the nature of the reaction and the way particles are created.

B. Spivak; F. Zhou

1994-01-01

352

Disagreement between theory and experiment in the simplest chemical reaction: Collision energy dependent rotational  

E-print Network

Disagreement between theory and experiment in the simplest chemical reaction: Collision energy, with the difference being more pronounced at higher collision energies. Disagreement between theory and experiment experimental rotational distributions for the reaction H D2HD( 3,j ) D at eight different collision energies

Zare, Richard N.

353

Direct Monte Carlo simulation of chemical reaction systems: Dissociation and recombination  

E-print Network

Direct Monte Carlo simulation of chemical reaction systems: Dissociation and recombination Shannon of Physics. I. INTRODUCTION In earlier studies1­5 we have found the direct Monte Carlo simulation method6 Monte Carlo simulation of dissociation-recombination reac- tions of the type M AB M A B. These reactions

Anderson, James B.

354

Nuclear spin selection rules in chemical reactions by angular momentum algebra  

E-print Network

Nuclear spin selection rules in chemical reactions by angular momentum algebra Takeshi Oka by Quack using molecular symmetry group are derived by using angu- lar momentum algebra. Instead: Selection rules; Nuclear spin modifications; Angular momentum algebra; Ion-neutral reactions; Molecular ions

Oka, Takeshi

355

Acidbase chemical reaction model for nucleation rates in the polluted atmospheric boundary layer  

E-print Network

Acid­base chemical reaction model for nucleation rates in the polluted atmospheric boundary layer for modeling nucleation rates, based on a sequence of acid­base reactions. The model uses empirical estimates, therefore, the earth's radiation budget. Measurements worldwide show that nucleation rates in the atmo

356

Effect of Finite-rate Chemical Reactions on Turbulence in Hypersonic Turbulent Boundary Layers  

E-print Network

Effect of Finite-rate Chemical Reactions on Turbulence in Hypersonic Turbulent Boundary Layers Lian and the reaction rate is typically temperature Ph.D. Student, Department of Mechanical and Aerospace Engineering simulations of reacting turbulent boundary layers to study the effects of finite-rate chemistry on turbulence

Martín, Pino

357

SHEAR LOCALIZATION AND CHEMICAL REACTION IN HIGH-STRAIN, HIGH-STRAIN-RATE DEFORMATION OF  

E-print Network

SHEAR LOCALIZATION AND CHEMICAL REACTION IN HIGH-STRAIN, HIGH-STRAIN-RATE DEFORMATION OF Ti.6±1 mm. Thermodynamic and kinetic calculations yield the reaction rate outside the shear bands.S.A. (Received 19 June 1997; accepted 19 December 1997) AbstractÐTi±Si mixtures were subjected to high-strain-rate

Meyers, Marc A.

358

THE JOURNAL OF CHEMICAL PHYSICS 134, 034313 (2011) Reaction of C2H2  

E-print Network

THE JOURNAL OF CHEMICAL PHYSICS 134, 034313 (2011) Reaction of C2H2 + (n · 2, m · 5) with NO2: Reaction on the singlet and triplet surfaces Jason M. Boyle, David M. Bell, and Scott L. Andersona) Department of Chemistry, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112

Anderson, Scott L.

2011-01-01

359

WATER RESOURCES RESEARCH, VOL. ???, XXXX, DOI:10.1029/, Simulation of chemical reaction via particle tracking  

E-print Network

. This becomes a convolution of the location densities of the two particles. The backward reaction is a simple Abstract. Chemical reactions may be simulated without regard to local concentrations by ap- plying simple. The formal connection is made by relating the probability den- sity of particle location and concentration (e

360

ABIOTIC REDUCTION REACTIONS OF ANTHROPOGENIC ORGANIC CHEMICALS IN ANAEROBIC SYSTEMS: A CRITICAL REVIEW  

EPA Science Inventory

The review is predicated upon the need for a detailed process-level understanding of factors influencing the reduction of anthropogenic organic chemicals in natural waters. In particular, abiotic reductions of anthropogenic organic chemicals are reviewed. The most important reduc...

361

The important role of reactions between atoms and radicals in flame propagation in cylinder reactors  

NASA Astrophysics Data System (ADS)

Results from studying features of the combustion of hydrogen in narrow cylinder reactors with diameters of <5.0 cm are presented. It is determined that the lower concentration limit of flame propagation in a reactor with a diameter of 1.1 cm is higher than 9% H2. The dependence of the principles of combustion on the method of initiation is studied. Mathematical simulations are performed that prove the impossibility of describing the combustion of hydrogen-deficient mixtures without assuming the cell structure of a flame. It is experimentally determined that increasing the termination of chains during the propagation of a flame results in its attenuation. It is noted that in addition to conductive heat abstraction, the recombination of atoms and radicals on the surface of the reactor plays an important role. It is concluded that these heterogeneous reactions representing the termination of reaction chains also result in the removal of recombination energy by the reactor's walls.

Azatyan, V. V.; Abramov, S. K.; Borisov, A. A.; Prokopenko, V. M.; Chapysheva, N. V.

2013-03-01

362

Femtosecond electron diffraction and spectroscopic studies of a solid state organic chemical reaction  

NASA Astrophysics Data System (ADS)

Photochromic diarylethene molecules are excellent model systems for studying electrocyclic reactions, in addition to having important technological applications in optoelectronics. The photoinduced ring-closing reaction in a crystalline photochromic diarylethene derivative was fully resolved using the complementary techniques of transient absorption spectroscopy and femtosecond electron crystallography. These studies are detailed in this thesis, together with the associated technical developments which enabled them. Importantly, the time-resolved crystallographic investigation reported here represents a highly significant proof-of-principle experiment. It constitutes the first study directly probing the molecular structural changes associated with an organic chemical reaction with sub-picosecond temporal and atomic spatial resolution---to follow the primary motions directing chemistry. In terms of technological development, the most important advance reported is the implementation of a radio frequency rebunching system capable of producing femtosecond electron pulses of exceptional brightness. The temporal resolution of this newly developed electron source was fully characterized using laser ponderomotive scattering, confirming a 435 +/- 75 fs instrument response time with 0.20 pC bunches. The ultrafast spectroscopic and crystallographic measurements were both achieved by exploiting the photoreversibility of diarylethene. The transient absorption study was first performed, after developing a novel robust acquisition scheme for thermally irreversible reactions in the solid state. It revealed the formation of an open-ring excited state intermediate, following photoexcitation of the open-ring isomer with an ultraviolet laser pulse, with a time constant of approximately 200 fs. The actual ring closing was found to occur from this intermediate with a time constant of 5.3 +/- 0.3 ps. The femtosecond diffraction measurements were then performed using multiple crystal orientations and a large number of different samples. To analyse the results, an innovative method was developed in which the apparently complex ring-closing reaction is distilled down to a small number of basic rotations. Immediately following photoexcitation, sub-picosecond structural changes associated with the formation of the intermediate are observed. The rotation of the thiophene rings is identified as the key motion. Subsequently, on the few picosecond time scale, the time-resolved diffraction patterns are observed to converge towards those associated with the closed-ring photoproduct. The formation of the closed-ring molecule is thus unambiguously witnessed.

Jean-Ruel, Hubert

363

Basics of Chemical Kinetics -1 Rate of reaction = rate of disappearance of A =  

E-print Network

Basics of Chemical Kinetics - 1 Rate of reaction = rate of disappearance of A = # of moles/volume) ; 1 mole = 6.023x1023 molecules rA = -k [A] rA = -k [A]2 rA = -k1 [A]/(1+k2[A]) Reaction rate law is an algebraic equation involving concentrations (not a differential equation) For a given reaction, the rate law

Albert, Réka

364

Investigation of electron density changes at the onset of a chemical reaction using the state-specific dual descriptor from conceptual density functional theory.  

PubMed

The electron density changes from reactants towards the transition state of a chemical reaction is expressed as a linear combination of the state-specific dual descriptors (SSDD) of the corresponding reactant complexes. Consequently, the SSDD can be expected to bear important resemblance to the so-called natural orbitals for chemical valence (NOCV), introduced as the orbitals that diagonalize the deformation density matrix of interacting molecules. This agreement is shown for three case studies: the complexation of a Lewis acid with a Lewis base, a SN2 nucleophilic substitution reaction and a Diels-Alder cycloaddition reaction. As such, the SSDD computed for reactant complexes are shown to provide important information about charge transfer interactions during a chemical reaction. PMID:25760399

De Proft, Frank; Forquet, Valérian; Ourri, Benjamin; Chermette, Henry; Geerlings, Paul; Morell, Christophe

2015-04-14

365

Theoretical studies of the dynamics of chemical reactions  

SciTech Connect

Recent research effort has focussed on several reactions pertinent to combustion. The formation of the formyl radical from atomic hydrogen and carbon monoxide, recombination of alkyl radicals and halo-alkyl radicals with halogen atoms, and the thermal dissociation of hydrogen cyanide and acetylene have been studied by modeling. In addition, the inelastic collisions of NCO with helium have been investigated.

Wagner, A.F. [Argonne National Laboratory, IL (United States)

1993-12-01

366

Theoretical Studies of Chemical Reactions following Electronic Excitation  

NASA Technical Reports Server (NTRS)

The use of multi-configurational wave functions is demonstrated for several processes: tautomerization reactions in the ground and excited states of the DNA base adenine, dissociation of glycine molecule after electronic excitation, and decomposition/deformation of novel rare gas molecules HRgF. These processes involve bond brealung/formation and require multi-configurational approaches that include dynamic correlation.

Chaban, Galina M.

2003-01-01

367

WORKSHOP ON STATUS OF TEST METHODS FOR ASSESSING POTENTIAL OF CHEMICALS TO INDUCE RESPIRATORY ALLERGIC REACTIONS  

EPA Science Inventory

Because of the association between allergy and asthma and the increasing incidence of morbidity and mortality due to asthma, there is growing concern over the potential of industrial chemicals to produce allergic reactions in the respiratory tract. Two classes of chemicals have b...

368

Direct and indirect effects of nitric oxide in chemical reactions relevant to biology  

Microsoft Academic Search

Categorization of the chemical reactions of NO into direct and indirect effects provides a framework to evaluate the role of NO in different biological situations. The diverse behavior of NO protecting against ROS toxicity yet potentiating the toxicity of other agents implies that the role of NO in each condition must be carefully evaluated. The chemical biology of NO can

David A. Wink; Matthew B. Grisham; James B. Mitchell; Peter C. Ford

1996-01-01

369

Reaction ensemble molecular dynamics: Direct simulation of the dynamic equilibrium properties of chemically reacting mixtures  

E-print Network

Reaction ensemble molecular dynamics: Direct simulation of the dynamic equilibrium properties of chemically reacting mixtures John K. Brennan,1, * Martin Lísal,2,3 Keith E. Gubbins,4 and Betsy M. Rice1 1 U 21005-5066, USA 2 E. Hála Laboratory of Thermodynamics, Institute of Chemical Process Fundamentals

Lisal, Martin

370

Rate constants for chemical reactions in high-temperature nonequilibrium air  

NASA Technical Reports Server (NTRS)

In the nonequilibrium atmospheric chemistry regime that will be encountered by the proposed Aeroassisted Orbital Transfer Vehicle in the upper atmosphere, where air density is too low for thermal and chemical equilibrium to be maintained, the detailed high temperature air chemistry plays a critical role in defining radiative and convective heating loads. Although vibrational and electronic temperatures remain low (less than 15,000 K), rotational and translational temperatures may reach 50,000 K. Attention is presently given to the effects of multiple temperatures on the magnitudes of various chemical reaction rate constants, for the cases of both bimolecular exchange reactions and collisional excitation and dissociation reactions.

Jaffe, R. L.

1986-01-01

371

Kinetics of Heterogeneous Chemical Reactions: A Theoretical Model for the Accumulation of Pesticides in Soil  

PubMed Central

A theoretical model for the accumulation of pesticides in soil has been proposed and discussed from the viewpoint of heterogeneous reaction kinetics with a basic aim to understand the complex nature of soil processes relating to the environmental pollution. In the bulk of soil, the pesticide disappears by diffusion and a chemical reaction; the rate processes considered on the surface of soil are diffusion, chemical reaction, vaporization, and regular pesticide application. The differential equations involved have been solved analytically by the Laplace-transform method. PMID:5279519

Lin, S. H.; Sahai, R.; Eyring, H.

1971-01-01

372

The mineralogic evolution of the Martian surface through time: Implications from chemical reaction path modeling studies  

NASA Technical Reports Server (NTRS)

Chemical reaction path calculations were used to model the minerals that might have formed at or near the Martian surface as a result of volcano or meteorite impact driven hydrothermal systems; weathering at the Martian surface during an early warm, wet climate; and near-zero or sub-zero C brine-regolith reactions in the current cold climate. Although the chemical reaction path calculations carried out do not define the exact mineralogical evolution of the Martian surface over time, they do place valuable geochemical constraints on the types of minerals that formed from an aqueous phase under various surficial and geochemically complex conditions.

Plumlee, G. S.; Ridley, W. I.; Debraal, J. D.; Reed, M. H.

1993-01-01

373

EFFICIENT CHEMICAL TRANSFORMATIONS USING ALTERNATIVE REACTION CONDITIONS AND MEDIA  

EPA Science Inventory

The diverse nature of chemical entities requires various green' strategic pathways in our quest towards attaining sustainability. A solvent-free approach that involves microwave (MW) exposure of neat reactants (undiluted) catalyzed by the surfaces of less-expensive and recyclable...

374

X-ray microspectroscopy and chemical reactions in soil microsites.  

PubMed

Soils provide long-term storage of environmental contaminants, which helps to protect water and air quality and diminishes negative impacts of contaminants on human and ecosystem health. Characterizing solid-phase chemical species in highly complex matrices is essential for developing principles that can be broadly applied to the wide range of notoriously heterogeneous soils occurring at the earth's surface. In the context of historical developments in soil analytical techniques, we describe applications of bulk-sample and spatially resolved synchrotron X-ray absorption spectroscopy (XAS) for characterizing chemical species of contaminants in soils, and for determining the uniqueness of trace-element reactivity in different soil microsites. Spatially resolved X-ray techniques provide opportunities for following chemical changes within soil microsites that serve as highly localized chemical micro- (or nano-)reactors of unique composition. An example of this microreactor concept is shown for micro-X-ray absorption near edge structure analysis of metal sulfide oxidation in a contaminated soil. One research challenge is to use information and principles developed from microscale soil chemistry for predicting macroscale and field-scale behavior of soil contaminants. PMID:21546654

Hesterberg, Dean; Duff, Martine C; Dixon, Joe B; Vepraskas, Michael J

2011-01-01

375

Control and detection of chemical reactions in microfluidic systems  

Microsoft Academic Search

Recent years have seen considerable progress in the development of microfabricated systems for use in the chemical and biological sciences. Much development has been driven by a need to perform rapid measurements on small sample volumes. However, at a more primary level, interest in miniaturized analytical systems has been stimulated by the fact that physical processes can be more easily

Andrew J. deMello

2006-01-01

376

Observation of a chemical reaction using a micromechanical sensor  

Microsoft Academic Search

We describe a new form of calorimeter designed for use in gaseous and vacuum environments which can sense chemical reac- tions with an estimated sensitivity limit of % 1 pJ. The device is based on a micromechanical Si lever coated with a thick layer of Al upon which a sample in the form of a thin layer is fixed or

J. K. Gimzewski; Ch. Gerber; E. Meyer; R. R. Schlittler

1994-01-01

377

The US Export-Import Bank: No evidence of financing restricted chemical exports to Iraq  

SciTech Connect

The authors have reviewed U.S. Export-Import Bank (Eximbank) transactions involving chemicals exported to Iraq from January 1987 to August 1990. Specifically, the authors examined (1) whether there was any evidence that the Eximbank financed the export of dual-use chemicals to Iraq and (2) what the Eximbank`s role was in monitoring and controlling chemical exports. Results are based primarily on a review of documents provided to them by the Eximbank. They did not verify the Eximbank data or corroborate it with the records of the banks or exporters involved in the transactions. There was no evidence in the documents they reviewed to suggest that the Eximbank financed the export of dual-use chemicals (chemicals with both commercial and military applications that could be used for chemical weapons) -- as defined by the Department of Commerce -- to Iraq between January 1987 and August 1990. There were approximately 190 transactions between Iraq and the Eximbank during this period. They focused their review on the eight transactions involving pesticides and related products. The Eximbank has no responsibility or authority for monitoring or controlling the export of chemicals or any other commodities: the Departments of Commerce and State and the US Customs Service share those responsibilities. Nevertheless, the Eximbank has recently developed specific procedures to review applications for financing chemical exports. However, such procedures were not in place when the Eximbank approved the applications for seven of eight pesticide transactions that occurred between January 1987 and August 1990.

Mendelowitz, A.I.; Watson, J.E.; Wood, S.E.; Logan, D.L.; Hinojosa, S.L.

1991-09-01

378

Chemical reaction and equilibration mechanisms in detonation waves  

Microsoft Academic Search

Experimental and theoretical evidence for the nonequilibrium Zeldovich-von Neumann-Doring (NEZND) theory of self-sustaining detonation is presented. High density, high temperature transition state theory is used to calculate unimolecular reaction rate constants for the initial decomposition of gaseous norbornene, liquid nitromethane, and solid, single crystal pentaerythritol tetranitrate as functions of shock temperature. The calculated rate constants are compared to those derived

Craig M. Tarver

1998-01-01

379

Chemical alterations taken place during deep-fat frying based on certain reaction products: a review.  

PubMed

Deep-fat frying at 180 °C or above is one of the most common food processing methods used for preparing of human kind foods worldwide. However, a serial of complex reactions such as oxidation, hydrolysis, isomerization, and polymerization take place during the deep-fat frying course and influence quality attributes of the final product such as flavor, texture, shelf life and nutrient composition. The influence of these reactions results from a number of their products including volatile compounds, hydrolysis products, oxidized triacylglycerol monomers, cyclic compounds, trans configuration compounds, polymers, sterol derivatives, nitrogen- and sulphur-containing heterocyclic compounds, acrylamide, etc. which are present in both frying oil and the fried food. In addition, these reactions are interacted and influenced by various impact factors such as frying oil type, frying conditions (time, temperature, fryer, etc.) and fried material type. Based on the published literatures, three main organic chemical reaction mechanisms namely hemolytic, heterolytic and concerted reaction were identified and supposed to elucidate the complex chemical alterations during deep-fat frying. However, well understanding the mechanisms of these reactions and their products under different conditions helps to control the deep-fat frying processing; therefore, producing healthy fried foods. By means of comprehensively consulting the papers which previously studied on the chemical changes occurred during deep-fat frying process, the major reaction products and corresponding chemical alterations were reviewed in this work. PMID:22800882

Zhang, Qing; Saleh, Ahmed S M; Chen, Jing; Shen, Qun

2012-09-01

380

A Statistical Model for Predicting Thermal Chemical Reaction Rate: Application to Bimolecule Reactions  

NASA Astrophysics Data System (ADS)

A model based on the statistics of individual atoms [Europhys. Lett. 94 (2011) 40002], which has been successfully applied to predict the rate constant of unimolecular reactions, was further extended to bimolecular reactions induced by collisions. Compared with the measured rate constants of the reactions S+SO2?SO+SO and NH3+Cl?NH2+HCl, the model is proved to be significantly better than conventional transition state theory. In order to strictly test the model, we perform molecular dynamics simulation of C60+C60?C120, and show that the rate constants are in excellent agreement with our model but far away from the transition state theory.

Li, Wang-Yao; Lin, Zheng-Zhe; Xu, Jian-Jun; Ning, Xi-Jing

2012-08-01

381

X-ray imaging of chemically active valence electrons during a pericyclic reaction  

PubMed Central

Time-resolved imaging of chemically active valence electron densities is a long-sought goal, as these electrons dictate the course of chemical reactions. However, X-ray scattering is always dominated by the core and inert valence electrons, making time-resolved X-ray imaging of chemically active valence electron densities extremely challenging. Here we demonstrate an effective and robust method, which emphasizes the information encoded in weakly scattered photons, to image chemically active valence electron densities. The degenerate Cope rearrangement of semibullvalene, a pericyclic reaction, is used as an example to visually illustrate our approach. Our work also provides experimental access to the long-standing problem of synchronous versus asynchronous bond formation and breaking during pericyclic reactions. PMID:25424639

Bredtmann, Timm; Ivanov, Misha; Dixit, Gopal

2014-01-01

382

Numerical Modeling of Coupled Variably-Saturated Fluid Flow and Reactive Transport with Fast and Slow Chemical Reactions  

SciTech Connect

The couplings among chemical reaction rates, advective and diffusive transport in fractured media or soils, and changes in hydraulic properties due to precipitation and dissolution within fractures and in rock matrix are important for both nuclear waste disposal and remediation of contaminated sites. This paper describes the development and application of LEHGC2.0, a mechanistically-based numerical model for simulation of coupled fluid flow and reactive chemical transport including both fast and slow reactions invariably saturated media. Theoretical bases and numerical implementations are summarized, and two example problems are demonstrated. The first example deals with the effect of precipitation-dissolution on fluid flow and matrix diffusion in a two-dimensional fractured media. Because of the precipitation and decreased diffusion of solute from the fracture into the matrix, retardation in the fractured medium is not as large as the case wherein interactions between chemical reactions and transport are not considered. The second example focuses on a complicated but realistic advective-dispersive-reactive transport problem. This example exemplifies the need for innovative numerical algorithms to solve problems involving stiff geochemical reactions.

LI, MING-HSU; SIEGEL, MALCOLM D.; YEH, GOUR-TSYH (GEORGE)

1999-09-20

383

Numerical modeling of coupled variably saturated fluid flow and reactive transport with fast and slow chemical reactions  

NASA Astrophysics Data System (ADS)

The couplings among chemical reaction rates, advective and diffusive transport in fractured media or soils, and changes in hydraulic properties due to precipitation and dissolution within fractures and in rock matrix are important for both nuclear waste disposal and remediation of contaminated sites. This paper describes the development and application of LEHGC2.0, a mechanistically based numerical model for simulation of coupled fluid flow and reactive chemical transport, including both fast and slow reactions in variably saturated media. Theoretical bases and numerical implementations are summarized, and two example problems are demonstrated. The first example deals with the effect of precipitation/dissolution on fluid flow and matrix diffusion in a two-dimensional fractured media. Because of the precipitation and decreased diffusion of solute from the fracture into the matrix, retardation in the fractured medium is not as large as the case wherein interactions between chemical reactions and transport are not considered. The second example focuses on a complicated but realistic advective-dispersive-reactive transport problem. This example exemplifies the need for innovative numerical algorithms to solve problems involving stiff geochemical reactions.

Yeh, Gour-Tsyh (George); Siegel, Malcolm D.; Li, Ming-Hsu

2001-02-01

384

Experimental results of exothermic reaction with concentration gradient catalyst in a solar chemical heat pump  

SciTech Connect

Solar chemical heat pump can upgrade the low temperature solar heat about 80 C to about 150--200 C by the reversible chemical reactions of 2-propanol/acetone/hydrogen, which are composed of endothermic and exothermic reactions. In the exothermic process of above reaction, a temperature peak occurs near the inlet of reaction zone in the case of arranging catalyst uniformly. Such a temperature distribution is not suitable for heat exchange. Therefore, the authors arrange the concentration of catalyst gradationally so as not to occur the temperature peak. In this paper, experimental results of exothermic reaction with concentration gradient catalyst in a double tubular exothermic reactor are presented. These results show that the arrangement of concentration gradient catalyst has the possibility about the temperature control in the catalytic reactor.

Takashima, Takumi; Doi, Takuya; Ando, Yuji; Tanaka, Tadayoshi [Electrotechnical Lab., Tsukuba, Ibaraki (Japan); Miyahara, Ryosuke; Kamoshida, Junji [Shibaura Inst. of Tech., Omiya, Saitama (Japan)

1997-12-31

385

Exploring chemical diversity via a modular reaction pairing strategy  

E-print Network

, distrib- ution, metabolism and excretion (ADME) properties were calculated by using the Volsurf program [39]. Cartesian grid- based chemical diversity analysis was performed according to the method described previously [40], by using standard H-aware 3D... to synthesize novel compounds for potential pharmacological applications it is critical that the com- pounds not only be unique but also be drug-like. Quantifying drug-likeness is one of the numerous methods that are regularly utilized as useful guidelines...

Loh, Joanna K.; Yoon, Sun Young; Samarakoon, Thiwanka Bandara; Rolfe, Alan; Porubsky, Patrick R.; Neuenswander, Benjamin; Lushington, Gerald H.; Hanson, Paul R.

2012-08-15

386

Journal of Chemical Ecology, Vol. 23, No. 4, 1997 REACTIONS OF Gammarus lacustris TO CHEMICAL  

E-print Network

and to chemical stimuli from two types of natural predators: dragonfly larvae (Aeshna eremita) and northern pike aquaria. However, no such trend was apparent in response to chemical stimuli from dragonfly larvae. The differences in response to chemical stimuli from pike and larval dragonflies suggest that G. lacustris does

Wisenden, Brian D.

387

Quantum chemical study of penicillin: Reactions after acylation  

NASA Astrophysics Data System (ADS)

The density functional theory methods were used on the model molecules of penicillin to determine the possible reactions after their acylation on ?-lactamase, and the results were compared with sulbactam we have studied. The results show that, the acylated-enzyme tetrahedral intermediate can evolves with opening of ?-lactam ring as well as the thiazole ring; the thiazole ring-open products may be formed via ?-lactam ring-open product or from tetrahedral intermediate directly. Those products, in imine or enamine form, can tautomerize via hydrogen migration. In virtue of the water-assisted, their energy barriers are obviously reduced.

Li, Rui; Feng, Dacheng; Zhu, Feng

388

The Modification of Biocellular Chemical Reactions by Environmental Physicochemicals  

NASA Astrophysics Data System (ADS)

Environmental risk factors affect human biological system to different extent from modification of biochemical reaction to cellular catastrophe. There are considerable public concerns about electromagnetic fields and endocrine disruptors. Their risk assessments have not been fully achieved because of their scientific uncertainty: electromagnetic fields just modify the bioreaction in the restricted cells and endocrine disruptors are quite unique in that their expression is dependent on the exposure periods throughout a life. Thus, we here describe their molecular characterization to establish the new risk assessments for environmental physicochemicals.

Ishido, M.

389

Derivatisation reactions in the chromatographic analysis of chemical warfare agents and their degradation products  

Microsoft Academic Search

The analysis of chemical warfare agents and their degradation products is an important component of verification of compliance with the Chemical Weapons Convention. Gas and liquid chromatography, particularly combined with mass spectrometry, are the major techniques used to detect and identify chemicals of concern to the Convention. The more polar analytes, and some of the more reactive or highly volatile

Robin M Black; Bob Muir

2003-01-01

390

Trust, but verify: On the importance of chemical structure curation in cheminformatics and QSAR modeling research  

PubMed Central

Molecular modelers and cheminformaticians typically analyze experimental data generated by other scientists. Consequently, when it comes to data accuracy, cheminformaticians are always at the mercy of data providers who may inadvertently publish (partially) erroneous data. Thus, dataset curation is crucial for any cheminformatics analysis such as similarity searching, clustering, QSAR modeling, virtual screening, etc., especially nowadays when the availability of chemical datasets in public domain has skyrocketed in recent years. Despite the obvious importance of this preliminary step in the computational analysis of any dataset, there appears to be no commonly accepted guidance or set of procedures for chemical data curation. The main objective of this paper is to emphasize the need for a standardized chemical data curation strategy that should be followed at the onset of any molecular modeling investigation. Herein, we discuss several simple but important steps for cleaning chemical records in a database including the removal of a fraction of the data that cannot be appropriately handled by conventional cheminformatics techniques. Such steps include the removal of inorganic and organometallic compounds, counterions, salts and mixtures; structure validation; ring aromatization; normalization of specific chemotypes; curation of tautomeric forms; and the deletion of duplicates. To emphasize the importance of data curation as a mandatory step in data analysis, we discuss several case studies where chemical curation of the original “raw” database enabled the successful modeling study (specifically, QSAR analysis) or resulted in a significant improvement of model's prediction accuracy. We also demonstrate that in some cases rigorously developed QSAR models could be even used to correct erroneous biological data associated with chemical compounds. We believe that good practices for curation of chemical records outlined in this paper will be of value to all scientists working in the fields of molecular modeling, cheminformatics, and QSAR studies. PMID:20572635

Fourches, Denis; Muratov, Eugene; Tropsha, Alexander

2010-01-01

391

Analysis of turbulent free-jet hydrogen-air diffusion flames with finite chemical reaction rates  

NASA Technical Reports Server (NTRS)

A numerical analysis is presented of the nonequilibrium flow field resulting from the turbulent mixing and combustion of an axisymmetric hydrogen jet in a supersonic parallel ambient air stream. The effective turbulent transport properties are determined by means of a two-equation model of turbulence. The finite-rate chemistry model considers eight elementary reactions among six chemical species: H, O, H2O, OH, O2 and H2. The governing set of nonlinear partial differential equations was solved by using an implicit finite-difference procedure. Radial distributions were obtained at two downstream locations for some important variables affecting the flow development, such as the turbulent kinetic energy and its dissipation rate. The results show that these variables attain their peak values on the axis of symmetry. The computed distribution of velocity, temperature, and mass fractions of the chemical species gives a complete description of the flow field. The numerical predictions were compared with two sets of experimental data. Good qualitative agreement was obtained.

Sislian, J. P.; Glass, I. I.; Evans, J. S.

1979-01-01

392

Benzene is an important industrial chemical (> 2 billion gallons produced annually in the  

E-print Network

Benzene is an important industrial chemical (> 2 billion gallons produced annually in the United leukemia (Snyder 2002). However, the mechanisms of benzene-induced hematotoxicity and leukemo- genesis further light on these mechanisms and better understand the risk benzene poses, we examined the effects

California at Berkeley, University of

393

Evaluation of commercially important chemical constituents in wild black pepper types  

Microsoft Academic Search

Matured black pepper berries from 8 wild types, on chemical screening, showed wide variations in commercially important constituents. The results have been compared with those of the cultivated varieties. The possibility of exploitation of this information in black pepper quality improvement programmes is discussed.

C. K. Mathai; P. M. Kumaran; K. C. Chandy

1980-01-01

394

Small-angle neutron scattering studies of chemical reaction and reaction-induced self-assembly  

NASA Astrophysics Data System (ADS)

We have investigated a self-assembling process of cellulose artificially synthesized via enzymatic polymerization by means of in-situ and time-resolved small-angle neutron scattering (SANS). The results elucidated the following: (i) cellulose molecules were synthesized at a special reaction site of the enzyme (cellulase) located on or near the smooth surface of the self-assembled enzymes formed in the reaction medium; (ii) the synthesized molecules associated themselves via diffusion-limited aggregation (DLA) and crystallized into fibrils and (iii) the fibrils formed the aggregates, which had the surface fractal dimension Ds increasing from 2 to 2.3 with the reaction time, on the smooth surface of the enzyme aggregates.

Tanaka, H.; Koizumi, S.; Hashimoto, T.; Kurosaki, K.; Ohmae, M.; Kobayashi, S.

2006-11-01

395

Charge exchange and chemical reactions with trapped thorium ions  

NASA Astrophysics Data System (ADS)

Most atomic nuclei have excitation energies ranging from keV to MeV. A unique exception is the ^229Th nucleus, which has an excited state just several eV above the nuclear ground state.ootnotetextB. R. Beck et al., Phys. Rev. Lett. 98, 142501 (2007). Th^3+ provides a convenient level structure for laser cooling in an rf Paul trap.ootnotetextC. J. Campbell et al., Phys. Rev. Lett 102, 233004 (2009). Unlike many ions commonly utilized in precision measurements, the trap lifetime of Th^3+ is limited to only several minutes. This is a severe limitation to experiments involving ^229Th as it is only available in minute quantities. Here we have studied the loss mechanisms by introduction of various contaminants and analyzed reaction products using trapped ion mass spectrometry techniques.ootnotetextL. R. Churchill et al., Phys. Rev. A 83, 012710 (2011).

Depalatis, Michael; Churchill, Layne; Chapman, Michael

2011-06-01

396

Heterogeneous nucleation - current transients under chemical reaction control  

E-print Network

Heterogeneous nucleation on catalytic surfaces plunged into a fluid is described through a stochastic model. To generate this non-equilibrium process we assume that the turn on of a electrostatic potential triggers a complex dynamics that includes a free Brownian motion, a reaction kinetic and a stimulated migration before the final adhesion of ions on the surface (electrode). At, when the potential is switched on, the spatial symmetry is broken and a two-stage process is developed. First the ion undergoes a change in its electrochemical character (at some region of the space) and then reacts at some specific points to stick together on the surface. The continuous addition of ions develops a material deposit connected to the current transient signals measured in electrochemical deposition processes. Unlike current models found in the literature, this procedure avoids the computation of the area covered by the diffusion zones, allowing a formalism skill to describe equally well the absorption of ions by channe...

D'Ajello-Tettamanzy, P C; Kipervaser, Z G S

2002-01-01

397

Chemically Activated Formation of Organic Acids in Reactions of the Criegee Intermediate with Aldehydes and Ketones  

SciTech Connect

Reactions of the Criegee intermediate (CI, .CH2OO.) are important in atmospheric ozonolysis models. In this work, we compute the rates for reactions between .CH2OO. and HCHO, CH3CHO and CH3COCH3 leading to the formation of secondary ozonides (SOZ) and organic acids. Relative to infinitely separated reactants, the SOZ in all three cases is found to be 48–51 kcal mol-1 lower in energy, formed via 1,3- cycloaddition of .CH2OO. across the CQO bond. The lowest energy pathway found for SOZ decomposition is intramolecular disproportionation of the singlet biradical intermediate formed from cleavage of the O–O bond to form hydroxyalkyl esters. These hydroxyalkyl esters undergo concerted decomposition providing a low energy pathway from SOZ to acids. Geometries and frequencies of all stationary points were obtained using the B3LYP/MG3S DFT model chemistry, and energies were refined using RCCSD(T)-F12a/cc-pVTZ-F12 single-point calculations. RRKM calculations were used to obtain microcanonical rate coefficients (k(E)) and the reservoir state method was used to obtain temperature and pressure dependent rate coefficients (k(T, P)) and product branching ratios. At atmospheric pressure, the yield of collisionally stabilized SOZ was found to increase in the order HCHO o CH3CHO o CH3COCH3 (the highest yield being 10-4 times lower than the initial .CH2OO. concentration). At low pressures, chemically activated formation of organic acids (formic acid in the case of HCHO and CH3COCH3, formic and acetic acid in the case of CH3CHO) was found to be the major product channel in agreement with recent direct measurements. Collisional energy transfer parameters and the barrier heights for SOZ reactions were found to be the most sensitive parameters determining SOZ and organic acid yield.

Jalan, Amrit; Allen, Joshua W.; Green, William H.

2013-08-08

398

Coupling quantum interpretative techniques: another look at chemical mechanisms in organic reactions  

PubMed Central

A cross ELF-NCI analysis is tested over prototypical organic reactions. The synergetic use of ELF and NCI enables the understanding of reaction mechanisms since each method can respectively identify regions of strong and weak electron pairing. Chemically intuitive results are recovered and enriched by the identification of new features. Non covalent interactions are found to foresee the evolution of the reaction from the initial steps. Within NCI, no topological catastrophe is observed as changes are continuous to such an extent that future reaction steps can be predicted from the evolution of the initial NCI critical points. Indeed, strong convergences through the reaction paths between ELF and NCI critical points enable to identify key interactions at the origin of the bond formation. VMD scripts enabling the automatic generation of movies depicting the cross NCI/ELF analysis along a reaction path (or following a Born-Oppenheimer molecular dynamics trajectory) are provided as S.I. PMID:23185140

Gillet, Natacha; Chaudret, Robin; Contreras-Garc?a, Julia; Yang, Weitao; Silvi, Bernard; Piquemal, Jean-Philip

2012-01-01

399

The Role of Comprehensive Detailed Chemical Kinetic Reaction Mechanisms in Combustion Research  

SciTech Connect

Recent developments by the authors in the field of comprehensive detailed chemical kinetic reaction mechanisms for hydrocarbon fuels are reviewed. Examples are given of how these mechanisms provide fundamental chemical insights into a range of combustion applications. Practical combustion consists primarily of chemical heat release from reactions between a fuel and an oxidizer, and computer simulations of practical combustion systems have become an essential tool of combustion research (Westbrook et al., 2005). At the heart of most combustion simulations, the chemical kinetic submodel frequently is the most detailed, complex and computationally costly part of a system model. Historically, the chemical submodel equations are solved using time-implicit numerical algorithms, due to the extreme stiffness of the coupled rate equations, with a computational cost that varies roughly with the cube of the number of chemical species in the model. While early mechanisms (c. 1980) for apparently simple fuels such as methane (Warnatz, 1980) or methanol (Westbrook and Dryer, 1979) included perhaps 25 species, current detailed mechanisms for much larger, more complex fuels such as hexadecane (Fournet et al., 2001; Ristori et al., 2001; Westbrook et al., 2008) or methyl ester methyl decanoate (Herbinet et al., 2008) have as many as 2000 or even 3000 species. Rapid growth in capabilities of modern computers has been an essential feature in this rapid growth in the size and complexity of chemical kinetic reaction mechanisms.

Westbrook, C K; Pitz, W J; Curran, H J; Mehl, M

2008-07-16

400

Detection of Medically Important Candida Species by Absolute Quantitation Real-Time Polymerase Chain Reaction  

PubMed Central

Background: The number of invasive candidiasis cases has risen especially with an increase in the number of immunosuppressed and immunocom promised patients. The early detection of Candida species which is specific and sensitive is important in determining the correct administration of antifungal drugs to patients. Objectives: This study aims to develop a method for the detection, identification and quantitation of medically important Candida species through quantitative polymerase chain reaction (qPCR). Materials and Methods: The isocitrate lyase (ICL) gene which is not found in mammals was chosen as the target gene of real-time PCR. Absolute quantitation of the gene copy number was achieved by constructing the plasmid containing the ICL gene which is used to generate standard curve. Twenty fungal species, two bacterial species and human DNA were tested to check the specificity of the detection method. Results: All eight Candida species were successfully detected, identified and quantitated based on the ICL gene. A seven-log range of the gene copy number and a minimum detection limit of 103 copies were achieved. Conclusions: A one-tube absolute quantification real-time PCR that differentiates medically important Candida species via individual unique melting temperature was achieved. Analytical sensitivity and specificity were not compromised. PMID:25789129

Than, Leslie Thian Lung; Chong, Pei Pei; Ng, Kee Peng; Seow, Heng Fong

2014-01-01

401

Mixing and chemical reaction in an idealized swirl chamber  

SciTech Connect

A vorticity-based, low-Mach-number model for simulating combustion in closed chambers is constructed. Numerical scheme is based on a mixed finite-difference pseudo-spectral discretization of the governing equations. Discrete evolution equations are integrated in time using a predictor-corrector scheme, while discrete elliptic systems are inverted with the help of fast-Poisson solver. Scheme is applied to analyze mixing and combustion in an idealized swirl cavity, which consists of the annular space between a spinning inner cylinder and a stationary reaction. To this end, we assume that the oxidizer and fuel are initially separated by a thin mixed region, and carefully control mixing levels by varying the duration of the swirl-driven mixing period. The mixture is then ignited along the boundary of the inner cylinder. When pre-mixing is complete, an axisymmetric flame front is established, and the reactants are consumed as the front propagates radially outwards. When the charge is partially mixed, combustion in the early stages predominantly occurs within a non-uniform premixed front. As this non-uniform front approaches the outer cylinder, a transition to a distributed combustion regime occurs. Following the transition, the remaining fuel burns at a slow rate within non-premixed flames which wrap around the inner cylinder. Results show that the mixing time has substantial effects on the pressure rise within the cavity and on the evolution of the burnt fraction, and that these effects become more pronounced as the Damkoehler number increases.

Knio, O.M.; Worlikar, A.S. [Johns Hopkins Univ., Baltimore, MD (United States). Dept. of Mechanical Engineering; Najm, H.N. [Sandia National Labs., Livermore, CA (United States)

1996-01-01

402

Changes in mechanical property of sandstone by chemical reaction  

Microsoft Academic Search

The effect of increasing temperature on the hydro-mechanical properties of rocks is a highly important topic. When we select a site for underground excavation for the disposal of high-level radioactive waste and CO2-underground storage, it is necessary to know the effects of temperature and confining pressure with water, called the deep geological environment. In fact increasing temperatures cause micro-cracking and

T. Takemura; M. Manaka; M. Takahashi

2006-01-01

403

Wear mechanisms in metal-on-metal bearings: the importance of tribochemical reaction layers.  

PubMed

Metal-on-metal (MoM) bearings are at the forefront in hip resurfacing arthroplasty. Because of their good wear characteristics and design flexibility, MoM bearings are gaining wider acceptance with market share reaching nearly 10% worldwide. However, concerns remain regarding potential detrimental effects of metal particulates and ion release. Growing evidence is emerging that the local cell response is related to the amount of debris generated by these bearing couples. Thus, an urgent clinical need exists to delineate the mechanisms of debris generation to further reduce wear and its adverse effects. In this study, we investigated the microstructural and chemical composition of the tribochemical reaction layers forming at the contacting surfaces of metallic bearings during sliding motion. Using X-ray photoelectron spectroscopy and transmission electron microscopy with coupled energy dispersive X-ray and electron energy loss spectroscopy, we found that the tribolayers are nanocrystalline in structure, and that they incorporate organic material stemming from the synovial fluid. This process, which has been termed "mechanical mixing," changes the bearing surface of the uppermost 50 to 200 nm from pure metallic to an organic composite material. It hinders direct metal contact (thus preventing adhesion) and limits wear. This novel finding of a mechanically mixed zone of nanocrystalline metal and organic constituents provides the basis for understanding particle release and may help in identifying new strategies to reduce MoM wear. PMID:19877285

Wimmer, Markus A; Fischer, Alfons; Büscher, Robin; Pourzal, Robin; Sprecher, Christoph; Hauert, Roland; Jacobs, Joshua J

2010-04-01

404

Analysis of parameter effects on chemical reaction coupled transport phenomena in SOFC anodes  

Microsoft Academic Search

Mass, heat and momentum transport processes are strongly coupled by internal chemical reforming reactions in planar design\\u000a solid oxide fuel cell (SOFC) anodes. In this paper, a three-dimensional computational fluid dynamics approach is applied to\\u000a simulate and analyze reforming reactions of methane and various transport processes in a duct relevant for SOFC anodes. The\\u000a results show that the anode duct

Jinliang Yuan; Yuan Huang; Bengt Sundén; Wei Guo Wang

2009-01-01

405

Structure in turbulent mixing layers and wakes using a chemical reaction  

Microsoft Academic Search

Plane turbulent mixing between two streams of water which contained dilute chemical reactants was studied in a new blow-down water tunnel. In a diffusion-limited reaction, a pH indicator, phenolphthalein, in one stream mixed and reacted with a base, sodium hydroxide, in the other stream to form a visible reaction product. The product was found to exist, as expected, in concentrated

R. Breidenthal

1981-01-01

406

Persistence of transition state structure in chemical reactions driven by fields oscillating in time  

E-print Network

Chemical reactions subjected to time-varying external forces cannot generally be described through a fixed bottleneck near the transition state barrier or dividing surface. A naive dividing surface attached to the instantaneous, but moving, barrier top also fails to be recrossing-free. We construct a moving dividing surface in phase space over a transition state trajectory. This surface is recrossing-free for both Hamiltonian and dissipative dynamics. This is confirmed even for strongly anharmonic barriers using simulation. The power of transition state theory is thereby applicable to chemical reactions and other activated processes even when the bottlenecks are time-dependent and move across space.

Galen T. Craven; Thomas Bartsch; Rigoberto Hernandez

2014-04-29

407

From graphite to porous carbon containing nanoparticles through chemical reactions  

SciTech Connect

Porous carbon containing large quantities of separated individual nanoparticles (2--100 nm) was produced. The chemical process includes fluorination or oxygenation of graphite, and then exposing the product (graphite fluoride or graphite oxide) to metal chlorides. The nanoparticles were metal halides or metal oxides, which could contain dopants if they were added during the synthesis process. The chlorides used in this research include those of Pd, Zn, Al and Li. Depending on the synthesis process, the carbon pores could be either filled with the nanoparticles, resulting in near-zero surface area and high metal concentration, or partially filled with nanoparticles, resulting in large surface areas. In this report, near zero surface areas were observed for a product of LiCl in carbon, and a surface area of 75 m{sup 2}/g was observed for the product of {gamma}-Al{sub 2}O{sub 3} in carbon. Heating these products in 1 atm air allowed the nanoparticles to become fused together in the form of metal oxide while the carbon was oxidized, producing metal oxides which have the same shape as the carbon precursors (fibers, fabrics, or powder) and large surface areas. These products are potentially useful in the area of batteries, high temperature gas sensors, and catalysts.

Hung, C.C. [NASA Lewis Research Center, Cleveland, OH (United States). Electro-Physics Branch; Corbin, J. [Cleveland State Univ., OH (United States). Dept. of Chemistry

1996-12-31

408

Child-Rearing Practices toward Children with Hemophilia: The Relative Importance of Clinical Characteristics and Parental Emotional Reactions.  

ERIC Educational Resources Information Center

Addresses the relative importance of clinical characteristics of the child and parental emotional reactions, to child-rearing practices towards children with hemophilia. Results indicate that mother's emotional reactions appear to have a stronger influence on child-rearing uncertainty and overprotection than clinical characteristics of the child.…

Banis, S.; Suurmeijer, Th. P. B. M.; van Peer, D. R.

1999-01-01

409

Reacting gas mixtures in the state-to-state approach: The chemical reaction rates  

NASA Astrophysics Data System (ADS)

In this work chemically reacting mixtures of viscous flows are analyzed within the framework of Boltzmann equation. By applying a modified Chapman-Enskog method to the system of Boltzmann equations general expressions for the rates of chemical reactions and vibrational energy transitions are determined as functions of two thermodynamic forces: the velocity divergence and the affinity. As an application chemically reacting mixtures of N2 across a shock wave are studied, where the first lowest vibrational states are taken into account. Here we consider only the contributions from the first four single quantum vibrational-translational energy transitions. It is shown that the contribution to the chemical reaction rate related to the affinity is much larger than that of the velocity divergence.

Kustova, Elena V.; Kremer, Gilberto M.

2014-12-01

410

Thursday, January 27, 2011 and Tuesday, February 1 These two lectures are on some basics of chemical reaction  

E-print Network

65). The reaction between CH4 and OH starts a whole reaction chain going that ultimately produces of chemical reaction rate theory, as applied to the atmosphere. This information is not found in many-71 (lifetimes and cycles) Chapter 9 ­ page 140-143 (see, especially, Table 9.1) #12;Bimolecular Reactions

Toohey, Darin W.

411

The reactions of proteins with carbohydrates: changes in nutritional value and chemical characteristics.  

E-print Network

mechanisms by which damage to the protein occurs during beating. Pirst, in the absence of carbohydrate, s reaction occurs between the free amino group of lysine and arginine, and the free carboxyl groups of glutsmic and aspertic acids. This blocking... acid or alkaline hydrolysis. The second, and most important type of damage occurs when the protein is heated in the presence of carbohydrate. In this reaction the protein is lowered in nutritive value by the reaction between the reducing group...

Jonsson, Haldor Turner

1961-01-01

412

Proposed chemical mechanisms leading to secondary organic aerosol in the reactions of aliphatic amines with hydroxyl and nitrate radicals  

NASA Astrophysics Data System (ADS)

The presence and importance of amines in the atmosphere, including aliphatic amines, continues to gain more attention. The atmospheric reaction mechanisms of these amines with key atmospheric radicals are important to predict both daytime and nighttime atmospheric chemistry. While previous studies have focused on the production of amine salts, this analysis looks at the importance of peroxy radical reactions to the formation of secondary organic aerosol. Atmospheric oxidation mechanisms are presented to explain the observed chemistry. A series of environmental chamber experiments were conducted in which aliphatic tertiary and secondary amines were reacted with either hydroxyl radical (OH) or nitrate radical (NO3). Chemical composition of the aerosol products was obtained with a High Resolution Time of Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and a Particle Into Liquid Sampler Time of Flight Mass Spectrometer (PILS-ToF-MS), while the chemical composition of the gas-phase products was obtained with a Selected Ion Flow Tube Mass Spectrometer (SIFT-MS). A number of aerosol-phase mass spectra showed highly oxidized fragments at a much higher molecular weight (MW) than the amine precursor. It is proposed that these larger compounds are oligomers formed through peroxy radical reactions with hydrogen rearrangement. Another reaction pathway observed was the formation of amine salts. The relative importance of each pathway to the overall production of aerosol is found to be dependent on the type of amine and oxidant. For example, the oligomers were observed in the tertiary methyl amines, while the formation of amine salts was more prevalent in the secondary and tertiary ethyl amines.

Price, Derek J.; Clark, Christopher H.; Tang, Xiaochen; Cocker, David R.; Purvis-Roberts, Kathleen L.; Silva, Philip J.

2014-10-01

413

Studies in photochemical smog chemistry. I. Atmospheric chemistry of toluene. II. Analysis of chemical reaction mechanisms for photochemical smog  

SciTech Connect

This study focuses on two related topics in the gas phase organic chemistry of importance in urban air pollution. Part I describes an experimental and modeling effort aimed at developing a new explicit reaction mechanism for the atmospheric photooxidation of toluene. This mechanism is tested using experimental data from both indoor and outdoor smog chamber facilities. The predictions of the new reaction mechanism are found to be in good agreement with both sets of experimental data. Additional simulations performed with the new mechanism are used to investigate various mechanistic paths, and to gain insight into areas where the understanding is not complete. The outdoor experimental facility, which was built to provide the second set of experimental data, consists of a 65 cubic meter teflon smog chamber together with full instrumentation capable of measuring ozone, nitrogen dioxide, nitric oxide, peroxyacetyl nitrate (PAN), carbon monoxide, relative humidity, temperature, aerosol size distributions, and of course toluene and its photooxidation products. In Part II, a theoretical analysis of lumped chemical reaction mechanisms for photochemical smog is presented. Included is a description of a new counter species analysis technique which can be used to analyze any complex chemical reaction mechanism. Finally, a new lumped mechanism for photochemical smog is developed and tested against experimental data from two smog chamber facilities. Advantages of this mechanism relative to the existing lumped mechanisms are discussed.

Leone, J.A.

1985-01-01

414

The reaction of aluminium clusters with water: a quantum chemical investigation  

NASA Astrophysics Data System (ADS)

The reaction of charged aluminium clusters with water in the gas phase is investigated theoretically. To this end, the doublet potential energy surface for the Aln+ + H2O ? AlnO+ + H2 reaction with n=6 is explored using density functional theory. The calculated reaction pathways include the initial adduct ion formation, the O-H bond activation and H2 elimination steps, and are consistent with the recent experimental report (Ref.2). The results of the current quantum-chemical study are relevant to the issues of catalytic role of the main group metal clusters and H2 generation.

Moc, Jerzy

2015-01-01

415

Equilibrium-like behavior in far-from-equilibrium chemical reaction networks  

E-print Network

In an equilibrium chemical reaction mixture, the number of molecules present obeys a Poisson distribution. We ask when the same is true of the steady state of a nonequilibrium reaction network and obtain an essentially complete answer. In particular, we show that networks with certain topological features must have a Poisson distribution, whatever the reaction rates. Such driven systems also obey an analog of the fluctuation-dissipation theorem. Our results may be relevant to biological systems and to the larger question of how equilibrium concepts might apply to nonequilibrium systems.

David K. Lubensky

2008-07-04

416

Ion-Molecule Reactions and Chemical Composition of Emanated from Herculane Spa Geothermal Sources  

PubMed Central

The paper presents a chemical composition analysis of the gases emanated from geothermal sources in the Herculane Spa area (Romania). The upper homologues of methane have been identified in these gases. An ion-molecule reaction mechanism could be implicated in the formation of the upper homologues of methane. The CH4+ ions that appear under the action of radiation are the starting point of these reactions. The presence of hydrogen in the emanated gases may be also a result of these reactions. PMID:19325844

Cosma, Constantin; Suciu, Ioan; Jäntschi, Lorentz; Bolboac?, Sorana D.

2008-01-01

417

Chemical rates in detonation reaction zones: Determination of temperature and pressure dependence  

Microsoft Academic Search

Bardo et al1 have pointed out that at detonalion pressure the volume of activation of a reaction can have an important effect on the reaction rate. It is shown that both the activation energy and activation volume cannot be determine from the experimental data2. (AIP)

John B. Bdzil; Ray Engelke

1982-01-01

418

O/S-1/ interactions - The product channels. [collisional electron quenching and chemical reaction pathway frequencies  

NASA Technical Reports Server (NTRS)

The first measurements are reported of the reaction pathways for the interaction between oxygen atoms in the 4.19 eV S-1 state, and four molecules, N2O, CO2, H2O, and NO. Distinction is made between three possible paths - quenching to O(D-1), quenching to O(P-3), and chemical reaction. With N2O, the most reasonable interpretation of the data indicates that there no reaction, in sharp contrast with the interaction between O(D-1) and N2O, which proceeds entirely by reaction. Similarly, there is no reaction with CO2. With H2O, the reactive pathway is the dominant one, although electronic quenching is not negligible. With NO, O(D-1) is the preferred product.

Slanger, T. G.; Black, G.

1978-01-01

419

Slow Hydrogen Transfer Reactions of Oxo— and Hydroxo— Vanadium Compounds: the Importance of Intrinsic Barriers  

PubMed Central

Reactions are described that interconvert vanadium(IV) oxo-hydroxo complexes [VIVO(OH)(R2bpy)2]BF4 (1a-c) and vanadium(V) dioxo complexes [VVO2(R2bpy)2]BF4 (2a-c) [R2bpy = 4,4?-di-t-butyl-2,2?-bipyridine (tBu2bpy), a; 4,4?-dimethyl-2,2?-bipyridine (Me2bpy), b; 2,2?-bipyridine (bpy), c]. These are rare examples of pairs of isolated, sterically unencumbered, first-row metal-oxo/hydroxo complexes that differ by a hydrogen atom (H+ + e?). The VIV– tBu2bpy derivative 1a has a useful 1H NMR spectrum, despite being paramagnetic. Complex 2a abstracts H• from organic substrates with weak O–H and C–H bonds, converting 2,6-tBu2-4-MeO-C6H2OH (ArOH) and 2,2,6,6-tetramethyl-N-hydroxy-piperidine (TEMPOH) to their corresponding radicals ArO• and TEMPO, hydroquinone to benzoquinone, and dihydroanthracene to anthracene. The equilibrium constant for 2a + ArOH ? 1a + ArO• is (4 ± 2) × 10?3, implying that the VO–H bond dissociation free energy (BDFE) is 70.6 ± 1.2 kcal mol?1. Consistent with this value, 1a is oxidized by 2,4,6-tBu3C6H2O•. All of these reactions are surprisingly slow, typically occurring over hours at ambient temperatures. The net hydrogen-atom pseudo-self-exchange 1a + 2b ? 2a + 1b, using the tBu- and Me-bpy substituents as labels, also occurs slowly, with kse = 1.3 × 10?2 M?1 s?1 at 298 K, ?H‡ = 15 ± 2 kcal mol?1, and ?S‡= 16 ± 5 cal mol?1 K. Using this kse and the BDFE, the vanadium reactions are shown to follow the Marcus cross relation moderately well, with calculated rate constants within 102 of the observed values. The vanadium self-exchange reaction is ca. 106 slower than that for the related RuIVO(py)(bpy)22+ / RuIIIOH(py)(bpy)22+ self-exchange. The origin of this dramatic difference has been probed with DFT calculations on the self-exchange reactions of 1c + 2c and on mono-cationic ruthenium complexes with pyrrolate or fluoride in place of the py ligands. The calculations reproduce the difference in barrier heights and show that transfer of a hydrogen atom involves more structural reorganization for vanadium than the Ru analogs. The vanadium complexes have larger changes in the metal–oxo and metal–hydroxo bond lengths, which is traced to the difference in d-orbital occupancy in the two systems. This study thus highlights the importance of intrinsic barriers in the transfer of a hydrogen atom, in addition to the thermochemical (bond strength) factors that have been previously emphasized. PMID:19292442

Waidmann, Christopher R.; Zhou, Xin; Tsai, Erin A.; Kaminsky, Werner; Hrovat, David A.; Borden, Weston Thatcher; Mayer, James M.

2009-01-01

420

A kinetic model for chemical reactions without barriers: transport coefficients and eigenmodes  

NASA Astrophysics Data System (ADS)

The kinetic model of the Boltzmann equation proposed in the work of Kremer and Soares 2009 for a binary mixture undergoing chemical reactions of symmetric type which occur without activation energy is revisited here, with the aim of investigating in detail the transport properties of the reactive mixture and the influence of the reaction process on the transport coefficients. Accordingly, the non-equilibrium solutions of the Boltzmann equations are determined through an expansion in Sonine polynomials up to the first order, using the Chapman-Enskog method, in a chemical regime for which the reaction process is close to its final equilibrium state. The non-equilibrium deviations are explicitly calculated for what concerns the thermal-diffusion ratio and coefficients of shear viscosity, diffusion and thermal conductivity. The theoretical and formal analysis developed in the present paper is complemented with some numerical simulations performed for different concentrations of reactants and products of the reaction as well as for both exothermic and endothermic chemical processes. The results reveal that chemical reactions without energy barrier can induce an appreciable influence on the transport properties of the mixture. Oppositely to the case of reactions with activation energy, the coefficients of shear viscosity and thermal conductivity become larger than those of an inert mixture when the reactions are exothermic. An application of the non-barrier model and its detailed transport picture are included in this paper, in order to investigate the dynamics of the local perturbations on the constituent number densities, and velocity and temperature of the whole mixture, induced by spontaneous internal fluctuations. It is shown that for the longitudinal disturbances there exist two hydrodynamic sound modes, one purely diffusive hydrodynamic mode and one kinetic mode.

Alves, Giselle M.; Kremer, Gilberto M.; Marques, Wilson, Jr.; Jacinta Soares, Ana

2011-03-01

421

Spectator Ions ARE Important! A Kinetic Study of the Copper-Aluminum Displacement Reaction  

ERIC Educational Resources Information Center

Surprisingly, spectator ions are responsible for unexpected kinetics in the biphasic copper(II)-aluminum displacement reaction, with the rate of reaction dependent on the identity of the otherwise ignored spectator ions. Application of a published kinetic analysis developed for a reaction between a rotating Al disk and a Cu(II) ion solution to the…

Sobel, Sabrina G.; Cohen, Skyler

2010-01-01

422

Electronically excited molecules: Reaction kinetics and emission of light: Nanosecond infrared spectroscopy, electronic emission from chemical reactions  

NASA Astrophysics Data System (ADS)

A time-resolved IR absorption spectrometer capable of detecting chemical transients on the nanosecond timescale was designed, constructed, and successfully implemented. The spectrometer was used to characterize the vibrational relaxation of an open shell radical species, CF3, produced with excess energy from the photolysis of the parent CF3I compound. The effects of vibrational excitation in the CF3 radical on the reaction CF3 + Br2 yields CF3Br + Br were measured. Broadband data collection techniques were used to monitor the reactive and relaxation pathways simultaneously. The energetic radicals react no faster than the thermalized CF3 and may actually have a lower cross section for reaction. The spectrometer was also used to detect the gas phase absorption spectra of the polyatomic radicals. A thorough investigation into ozone-olefin reactions in a cryogenic matrix environment was completed. It was possible to complex ozone with various olefinic partners through careful control of the matrix deposition process, despite the very low (1-5 kcal/mole) activation energies for the ozonolysis reactions. The ground state complexes were observed to form a charge-transfer (CT) complex upon excitation.

Moore, C. B.

1992-02-01

423

Super-Resolution Fingerprinting Detects Chemical Reactions and Idiosyncrasies of Single DNA Pegboards  

E-print Network

Super-Resolution Fingerprinting Detects Chemical Reactions and Idiosyncrasies of Single DNA in nanoscale topography (PAINT) using site-specific DNA probes to acquire two-dimensional density maps of specific features patterned on nanoscale DNA origami pegboards. We show that PAINT has a localization

Walter, Nils G.

424

Detailed chemical kinetic reaction mechanisms for soy and rapeseed biodiesel fuels  

Microsoft Academic Search

A detailed chemical kinetic reaction mechanism is developed for the five major components of soy biodiesel and rapeseed biodiesel fuels. These components, methyl stearate, methyl oleate, methyl linoleate, methyl linolenate, and methyl palmitate, are large methyl ester molecules, some with carboncarbon double bonds, and kinetic mechanisms for them as a family of fuels have not previously been available. Of particular

C. K. Westbrook; C. V. Naik; O. Herbinet; W. J. Pitz; M. Mehl; S. M. Sarathy; H. J. Curran

2011-01-01

425

THE JOURNAL OF CHEMICAL PHYSICS 134, 244702 (2011) Reaction of aluminum clusters with water  

E-print Network

THE JOURNAL OF CHEMICAL PHYSICS 134, 244702 (2011) Reaction of aluminum clusters with water Satoshi) The atomistic mechanism of rapid hydrogen production from water by an aluminum cluster is inves- tigated by ab-bond switching events similar to the Grotthuss mechanism, where hydroxide ions are converted to water molecules

Southern California, University of

2011-01-01

426

Chemical reaction of metal-fullerene in gas phase (2) >Masamichi Konoa  

E-print Network

.of Tokyo Since the discovery of macroscopic generation and purification procedure of fullerenes intriguing issues. Recently X-ray diffraction study[1] demonstrated that the metal atom(s) are encapsulated. Throughout the study, no chemical reaction of NO molecules and H atom were observed for LaCn- where n

Maruyama, Shigeo

427

Effectiveness of Conceptual Change Text-Oriented Instruction on Students' Understanding of Energy in Chemical Reactions  

ERIC Educational Resources Information Center

The aim of this study is to compare the effectiveness of conceptual change text instruction (CCT) in the context of energy in chemical reactions. The subjects of the study were 60, 10th grade students at a high school, who were in two different classes and taught by the same teacher. One of the classes was randomly selected as the experimental…

Tastan, Ozgecan; Yalcinkaya, Eylem; Boz, Yezdan

2008-01-01

428

Using Drawing Technology to Assess Students' Visualizations of Chemical Reaction Processes  

ERIC Educational Resources Information Center

In this study, we investigated how students used a drawing tool to visualize their ideas of chemical reaction processes. We interviewed 30 students using thinking-aloud and retrospective methods and provided them with a drawing tool. We identified four types of connections the students made as they used the tool: drawing on existing knowledge,…

Chang, Hsin-Yi; Quintana, Chris; Krajcik, Joseph

2014-01-01

429

Speed faults in computation by chemical reaction networks Ho-Lin Chen  

E-print Network

Speed faults in computation by chemical reaction networks Ho-Lin Chen Rachel Cummings David Doty fields as diverse as synthetic biology, nanotechnology, and computer science. In these efforts we equivalent to population protocols, a model of distributed computing introduced by Angluin, Aspnes, Diamadi

Soloveichik, David

430

Reaction and diffusion kinetics during the initial stages of isothermal chemical vapor infiltration  

Microsoft Academic Search

This paper reports that individual bundles of ceramic fibers were infiltrated with SiC to study the reaction and diffusion kinetics during isothermal chemical vapor infiltration (CVI). More uniform infiltration was observed in samples where baffles were placed in the reactor and when HC1 was added to the inlet gases. The evolution of the microstructure was modeled using an analytical expression

Brian W. Sheldon; Theodore M. Besmann

1991-01-01

431

Numerical Simulation of Superfast Shock-Induced Chemical Reaction in Titanium - Silicon Mixture  

SciTech Connect

A phenomenological zeroth-order kinetic model for computations of shock-induced solid-state chemical reactions in porous mixtures is proposed. In the model a porous mixture is considered as a continuous medium whose thermomechanical properties are determined at each time step depending on mass fractions of the components. The kinetic relationships are characterized by a constant rate of chemical transformation under shock wave loading. The heat release due to chemical transformation is introduced in the energy equation. The effect of the dispersity of the mixture components on the reaction rate is taken into account by varying the constants that enter the kinetic model. The results of the numerical computations for porous Ti-Si mixture reflect the fact that the process can be divided into several stages (dynamic compaction, shock-wave propagation, reaction of synthesis). It is shown that an increase in the chemical-reaction rate can give rise to flow regimes in which the unloading wave almost stops.

Zelepugin, S. A. [Dept. for Structural Macrokinetics, Tomsk Scientific Centre, SD RAS, Tomsk, 634021 (Russian Federation); Nikulichev, V. B. [Kyrgyz-Russian Slavonic University, Bishkek, 720000 (Kyrgyzstan); Ivanova, O. V. [Tomsk State University, Tomsk, 634050 (Russian Federation)

2006-07-28

432

Temperature-compensated chemical reactions Kanaka Rajan and L. F. Abbott  

E-print Network

Temperature-compensated chemical reactions Kanaka Rajan and L. F. Abbott Center for Neurobiology of these rhythms is that their periods remain fairly constant over a wide range of physiological temperatures, a feature called temperature compensation. Although circadian rhythms have been associated with periodic

Columbia University

433

Excess Energy from Chemical Reactions of Water (H 2 O and\\/or D 2 O)  

Microsoft Academic Search

Some combustible substance has been found from chemical reactions of water (H 2 O and\\/or D 2 O) using methods of electrolysis and catalysis. Experimental results indicate that the combustible substance was generated through transferring electrons from water and it is shown dual properties of inorganic water and organic compounds. The oxidation, including combustion and wet oxidation, of this combustible

Fu Liu

434

Constant centrifugal potential approximation for atom-diatom chemical reaction dynamics  

E-print Network

Constant centrifugal potential approximation for atom-diatom chemical reaction dynamics Kengo,Myodaiji, Okazaki 444. Japan (Received 28 September 1993; accepted 8 December 1993) The constant centrifugal of such practically useful approxima- tions the constant centrifugal potential approximation (CCPA) (or the energy

Takada, Shoji

435

THE REACTION RATE OF HYDROGEN AND DEUTERIUM AS MEASURED IN A CHEMICAL QUENCH TUBE  

Microsoft Academic Search

Reprinted from thesis dated June 1962. A chemical quench tube was built ; and instrumented in such a way that the reaction rate of hydrogen and deuterium ; (Hâ + Dâ in equilibrium 2 HD) could be measured. The main ; innovation was the use of a solenoid-actuated valve to isolate the reacting gas ; and prevent it from mixing

1962-01-01

436

The Effective Concepts on Students' Understanding of Chemical Reactions and Energy  

ERIC Educational Resources Information Center

The purpose of this study was to determine the relationship between the basic concepts related to the unit of "Chemical Reactions and Energy" and the sub-concepts underlying for meaningful learning of the unit and to investigate the effectiveness of them on students' learning achievements. For this purpose, the basic concepts of the unit were…

Ayyildiz, Yildizay; Tarhan, Leman

2012-01-01

437

Facilitating High School Students' Use of Multiple Representations to Describe and Explain Simple Chemical Reactions  

ERIC Educational Resources Information Center

This study involved the evaluation of the efficacy of a planned instructional program to facilitate understanding of the macroscopic, submicroscopic and symbolic representational systems when describing and explaining chemical reactions by sixty-five Grade 9 students in a Singapore secondary school. A two-tier multiple-choice diagnostic instrument…

Chandrasegaran, A. L.; Treagust, David F.; Mocerino, Mauro

2011-01-01

438

Two Experiments to Approach the Boltzmann Factor: Chemical Reaction and Viscous Flow  

ERIC Educational Resources Information Center

In this paper we discuss a pedagogical approach aimed at pointing out the role played by the Boltzmann factor in describing phenomena usually perceived as regulated by different mechanisms of functioning. Experimental results regarding some aspects of a chemical reaction and of the viscous flow of some liquids are analysed and described in terms…

Fazio, Claudio; Battaglia, Onofrio R.; Guastella, Ivan

2012-01-01

439

Chemical accelerator studies of reaction dynamics: Ar^+ + CH4 ? ArH^+ + CH3  

E-print Network

Chemical accelerator studies on isotopic variants of the reaction Ar+ + CH4 ? ArH+ + CH3 are reported. Velocity and angular distributions of the ionic product as a function of initial translational energy have been measured over the energy range 0...

Wyatt, J. R.; Strattan, L. W.; Snyder, S. C.; Hierl, Peter M.

1975-01-01

440

Turkish, Indian, and American Chemistry Textbooks Use of Inscriptions to Represent "Types of Chemical Reactions"  

ERIC Educational Resources Information Center

The purpose of this study was to investigate inscriptions used in "Types of Chemical Reactions" topic in Turkish, Indian, and American chemistry textbooks. We investigated both the types of inscriptions and how they were used in textbooks to support learning. A conceptual analysis method was employed to determine how those textbooks use…

Aydin, Sevgi; Sinha, Somnath; Izci, Kemal; Volkmann, Mark

2014-01-01

441

DEVELOPMENT AND TESTING OF A SURROGATE SPECIES CHEMICAL REACTION MECHANISM. VOLUME 2  

EPA Science Inventory

A photochemical reaction mechanism has been updated and extensively evaluated. The testing and refinement of the surrogate species mechanism was performed in order to create an improved chemical mechanism for the atmospheric simulation models that are used to develop ozone contro...

442

DEVELOPMENT AND TESTING OF A SURROGATE SPECIES CHEMICAL REACTION MECHANISM. VOLUME 1  

EPA Science Inventory

A photochemical reaction mechanism has been updated and extensively evaluated. The testing and refinement of the surrogate species mechanism was performed in order to create an improved chemical mechanism for the atmospheric simulation models that are used to develop ozone contro...

443

General chemical kinetics computer program for static and flow reactions, with application to combustion and shock-tube kinetics  

NASA Technical Reports Server (NTRS)

A general chemical kinetics program is described for complex, homogeneous ideal-gas reactions in any chemical system. Its main features are flexibility and convenience in treating many different reaction conditions. The program solves numerically the differential equations describing complex reaction in either a static system or one-dimensional inviscid flow. Applications include ignition and combustion, shock wave reactions, and general reactions in a flowing or static system. An implicit numerical solution method is used which works efficiently for the extreme conditions of a very slow or a very fast reaction. The theory is described, and the computer program and users' manual are included.

Bittker, D. A.; Scullin, V. J.

1972-01-01

444

Stability of a laminar premixed supersonic free shear layer with chemical reactions  

NASA Technical Reports Server (NTRS)

The stability of a two-dimensional compressible supersonic flow in the wake of a flat plate is discussed. The fluid is a multi-species mixture which is undergoing finite rate chemical reactions. The spatial stability of an infinitesimal disturbance in the fluid is considered. Numerical solutions of the eigenvalue stability equations for both reactive and nonreactive supersonic flows are presented and discussed. The chemical reactions have significant influence on the stability behavior. For instance, a neutral eigenvalue is observed near the freestream Mach number of 2.375 for the nonreactive case, but disappears when the reaction is turned on. For reactive flows, the eigenvalues are not very dependent on the free stream Mach number.

Menon, S.; Anderson, J. D., Jr.; Pai, S. I.

1984-01-01

445

Progress toward chemical accuracy in the computer simulation of condensed phase reactions.  

PubMed Central

We describe a procedure for the generation of chemically accurate computer-simulation models to study chemical reactions in the condensed phase. The process involves (i) the use of a coupled semiempirical quantum and classical molecular mechanics method to represent solutes and solvent, respectively; (ii) the optimization of semiempirical quantum mechanics (QM) parameters to produce a computationally efficient and chemically accurate QM model; (iii) the calibration of a quantum/classical microsolvation model using ab initio quantum theory; and (iv) the use of statistical mechanical principles and methods to simulate, on massively parallel computers, the thermodynamic properties of chemical reactions in aqueous solution. The utility of this process is demonstrated by the calculation of the enthalpy of reaction in vacuum and free energy change in aqueous solution for a proton transfer involving methanol, methoxide, imidazole, and imidazolium, which are functional groups involved with proton transfers in many biochemical systems. An optimized semiempirical QM model is produced, which results in the calculation of heats of formation of the above chemical species to within 1.0 kcal/mol (1 kcal = 4.18 kJ) of experimental values. The use of the calibrated QM and microsolvation QM/MM (molecular mechanics) models for the simulation of a proton transfer in aqueous solution gives a calculated free energy that is within 1.0 kcal/mol (12.2 calculated vs. 12.8 experimental) of a value estimated from experimental pKa values of the reacting species. PMID:11607654

Bash, P A; Ho, L L; MacKerell, A D; Levine, D; Hallstrom, P

1996-01-01

446

Looking for chemical reaction networks exhibiting a drift along a manifold of marginally stable states.  

PubMed

I recently reported some examples of mass-action equations that have a continuous manifold of marginally stable stationary states [Brogioli, D., 2010. Marginally stable chemical systems as precursors of life. Phys. Rev. Lett. 105, 058102; Brogioli, D., 2011. Marginal stability in chemical systems and its relevance in the origin of life. Phys. Rev. E 84, 031931]. The corresponding chemical reaction networks show nonclassical effects, i.e. a violation of the mass-action equations, under the effect of the concentration fluctuations: the chemical system drifts along the marginally stable states. I proposed that this effect is potentially involved in abiogenesis. In the present paper, I analyze the mathematical properties of mass-action equations of marginally stable chemical reaction networks. The marginal stability implies that the mass-action equations obey some conservation law; I show that the mathematical properties of the conserved quantity characterize the motion along the marginally stable stationary state manifold, i.e. they allow to predict if the fluctuations give rise to a random walk or a drift under the effect of concentration fluctuations. Moreover, I show that the presence of the drift along the manifold of marginally stable stationary-states is a critical property, i.e. at least one of the reaction constants must be fine tuned in order to obtain the drift. PMID:23160143

Brogioli, Doriano

2013-02-01

447

Observing metal-catalyzed chemical reactions in situ using surface-enhanced Raman spectroscopy on Pd-Au nanoshells.  

PubMed

Insight into the nature of transient reaction intermediates and mechanistic pathways involved in heterogeneously catalyzed chemical reactions is obtainable from a number of surface spectroscopic techniques. Carrying out these investigations under actual reaction conditions is preferred but remains challenging, especially for catalytic reactions that occur in water. Here, we report the direct spectroscopic study of the catalytic hydrodechlorination of 1,1-dichloroethene in H2O using surface-enhanced Raman spectroscopy (SERS). With Pd islands grown on Au nanoshell films, this reaction can be followed in situ using SERS, exploiting the high enhancements and large active area of Au nanoshell SERS substrates, the transparency of Raman spectroscopy to aqueous solvents, and the catalytic activity enhancement of Pd by the underlying Au metal. The formation and subsequent transformation of several adsorbate species was observed. These results provide the first direct evidence of the room-temperature catalytic hydrodechlorination of a chlorinated solvent, a potentially important pathway for groundwater cleanup, as a sequence of dechlorination and hydrogenation steps. More broadly, the results highlight the exciting prospects of studying catalytic processes in water in situ, like those involved in biomass conversion and proton-exchange membrane fuel cells. PMID:19554693

Heck, Kimberly N; Janesko, Benjamin G; Scuseria, Gustavo E; Halas, Naomi J; Wong, Michael S

2008-12-10

448

Chemical kinetic analysis of hydrogen-air ignition and reaction times  

NASA Technical Reports Server (NTRS)

An anaytical study of hydrogen air kinetics was performed. Calculations were made over a range of pressure from 0.2 to 4.0 atm, temperatures from 850 to 2000 K, and mixture equivalence ratios from 0.2 to 2.0. The finite rate chemistry model included 60 reactions in 20 species of the H2-O2-N2 system. The calculations also included an assessment of how small amounts of the chemicals H2O, NOx, H2O2, and O3 in the initial mixture affect ignition and reaction times, and how the variation of the third body efficiency of H2O relative of N2 in certain key reactions may affect reaction time. The results indicate that for mixture equivalence ratios between 0.5 and 1.7, ignition times are nearly constant; however, the presence of H2O and NO can have significant effects on ignition times, depending on the mixture temperature. Reaction time is dominantly influenced by pressure but is nearly independent of initial temperature, equivalence ratio, and the addition of chemicals. Effects of kinetics on reaction at supersonic combustor conditions are discussed.

Rogers, R. C.; Schexnayder, C. J., Jr.

1981-01-01

449

Balanced biochemical reactions: a new approach to unify chemical and biochemical thermodynamics.  

PubMed

A novel procedure is presented which, by balancing elements and electric charge of biochemical reactions which occur at constant pH and pMg, allows assessing the thermodynamics properties of reaction ?(r)G'?, ?(r)H'?, ?(r)S'? and the change in binding of hydrogen and magnesium ions of these reactions. This procedure of general applicability avoids the complex calculations required by the use of the Legendre transformed thermodynamic properties of formation ?(f)G'?, ?(f)H'? and ?(f)S'? hitherto considered an obligatory prerequisite to deal with the thermodynamics of biochemical reactions. As a consequence, the term "conditional" is proposed in substitution of "Legendre transformed" to indicate these thermodynamics properties. It is also shown that the thermodynamic potential G is fully adequate to give a criterion of spontaneous chemical change for all biochemical reactions and then that the use of the Legendre transformed G' is unnecessary. The procedure proposed can be applied to any biochemical reaction, making possible to re-unify the two worlds of chemical and biochemical thermodynamics, which so far have been treated separately. PMID:22247780

Sabatini, Antonio; Vacca, Alberto; Iotti, Stefano

2012-01-01

450

Quantum chemical approach to study the spectral properties of some important precursor of bio-molecules  

NASA Astrophysics Data System (ADS)

This work reports the spectral information of some interstellar complex molecules which could be treated as the precursor molecules for the formation of some bio-molecules in the interstellar medium (ISM). We carry out quantum chemical simulation to consider the Core correlation and vibrational corrections to the rotational constants and centrifugal distortion constants which are computed from harmonic and anharmonic force fields obtained at MP2/6-311G(d,p) level of theory. These precursor molecules could be produced in the gas phase as well as in the ice phase. This prompted use to couple the hydrodynamics of the collapsing phase of the protostar with our reasonably large chemical network including the gas phase as well as the grain phase chemical network, to study the chemical evolution of these species during the collapsing phase of a proto-star. We have noticed the Significant differences between spectroscopy of these species in the gas as well as in ice (water ice) phase due to the solute-solvent elctrostatic interactions. Time dependent density functional theory (TDDFT) is used to study the UV-VIS spectrum of these complexmolecules which are biologically important. Interstellar grain mantle around the dense cloud (> 104 cm-3) mainly composed by 60-70% Water, 5-30% Methanol and 2-20% CO2. So in reality, the ice could be mixed instead of simple water ice. To have an idea about the real spectra, we carry out our simulations for the mixed ice also by considering the actual composition of the above species in interstellar grain mantle. Spectral signatures are found to be significantly shifted with change of the solvent which confirms that the polarization of the solute by the continum has important effects on the absolute and relative solvation energies. Our simulated spectrum are in good agreement with some of the recent experimental result. We expect that our quantum chemical approach along with the hydro-chemical study might be useful for the observer to predict the abundances of some bio-molecules based on the chemical abundances of their precursor molecules.

Majumdar, Liton; Das, Ankan; Chakrabarti, Sandip K.; Chakrabarti, Sonali

2013-06-01

451

Chemical mixtures from a public health perspective: the importance of research for informed decision making.  

PubMed

When considered from a public health perspective, the central question regarding chemical mixtures is deceptively simple: Are current approaches to risk assessment for chemical mixtures affording effective (adequate) and efficient (cost-effective) protection for members of our society? Answering this question realistically depends on an understanding of the hierarchical goals of public health (i.e. prevention, intervention, treatment) and an accurate evaluation of the extent to which these goals are being achieved. To allow decision makers to make informed judgments about the health risks of chemical mixtures, adequate scientific knowledge and understanding must be available to support risk assessment activities, which are an integral part of the regulatory decision making process. Designing and implementing relevant research depends on the existence of a feedback loop between researchers and regulators, where the information needs of regulators influence the nature and direction of research and the information and understanding generated by researchers improves the scientific basis for public health decisions. A clear, consistent, commonly accepted taxonomy for describing important mixture-related phenomena is a key factor in creating and maintaining the necessary feedback loop. Ultimately, both researchers and regulators share a common goal with regard to chemical mixtures; improving the state-of-the-science so that we can make informed decisions about protecting public health. A survey of research issues and needs that are crucial to attaining this goal is presented. PMID:8571378

Sexton, K; Beck, B D; Bingham, E; Brain, J D; DeMarini, D M; Hertzberg, R C; O'Flaherty, E J; Pounds, J G

1995-12-28

452

Chemical Kinetic Reaction Mechanisms for Combustion of Hydrocarbon and Other Types of Chemical Fuels  

DOE Data Explorer

Reaction mechanisms have been tested and validated extensively through comparisons between computed results and measured data from laboratory experiments (e.g., shock tubes, laminar flames, rapid compression machines, flow reactors, stirred reactors) and from practical systems (e.g., diesel engines, spark-ignition engines, homogeneous charge, compression ignition (HCCI) engines). These kinetic models are used to examine a wide range of combustion systems.

453

Adaptively biased sequential importance sampling for rare events in reaction networks with comparison to exact solutions from finite buffer dCME method  

NASA Astrophysics Data System (ADS)

Critical events that occur rarely in biological processes are of great importance, but are challenging to study using Monte Carlo simulation. By introducing biases to reaction selection and reaction rates, weighted stochastic simulation algorithms based on importance sampling allow rare events to be sampled more effectively. However, existing methods do not address the important issue of barrier crossing, which often arises from multistable networks and systems with complex probability landscape. In addition, the proliferation of parameters and the associated computing cost pose significant problems. Here we introduce a general theoretical framework for obtaining optimized biases in sampling individual reactions for estimating probabilities of rare events. We further describe a practical algorithm called adaptively biased sequential importance sampling (ABSIS) method for efficient probability estimation. By adopting a look-ahead strategy and by enumerating short paths from the current state, we estimate the reaction-specific and state-specific forward and backward moving probabilities of the system, which are then used to bias reaction selections. The ABSIS algorithm can automatically detect barrier-crossing regions, and can adjust bias adaptively at different steps of the sampling process, with bias determined by the outcome of exhaustively generated short paths. In addition, there are only two bias parameters to be determined, regardless of the number of the reactions and the complexity of the network. We have applied the ABSIS method to four biochemical networks: the birth-death process, the reversible isomerization, the bistable Schlögl model, and the enzymatic futile cycle model. For comparison, we have also applied the finite buffer discrete chemical master equation (dCME) method recently developed to obtain exact numerical solutions of the underlying discrete chemical master equations of these problems. This allows us to assess sampling results objectively by comparing simulation results with true answers. Overall, ABSIS can accurately and efficiently estimate rare event probabilities for all examples, often with smaller variance than other importance sampling algorithms. The ABSIS method is general and can be applied to study rare events of other stochastic networks with complex probability landscape.

Cao, Youfang; Liang, Jie

2013-07-01

454

Method and apparatus for obtaining enhanced production rate of thermal chemical reactions  

DOEpatents

The present invention is a method and apparatus (vessel) for providing a heat transfer rate from a reaction chamber through a wall to a heat transfer chamber substantially matching a local heat transfer rate of a catalytic thermal chemical reaction. The key to the invention is a thermal distance defined on a cross sectional plane through the vessel inclusive of a heat transfer chamber, reaction chamber and a wall between the chambers. The cross sectional plane is perpendicular to a bulk flow direction of the reactant stream, and the thermal distance is a distance between a coolest position and a hottest position on the cross sectional plane. The thermal distance is of a length wherein the heat transfer rate from the reaction chamber to the heat transfer chamber substantially matches the local heat transfer rate.

Tonkovich, Anna Lee Y [Pasco, WA; Wang, Yong [Richland, WA; Wegeng, Robert S [Richland, WA; Gao, Yufei [Kennewick, WA

2003-04-01

455

Middle atmosphere heating by exothermic chemical reactions involving odd-hydrogen species  

NASA Technical Reports Server (NTRS)

The rate of heating which occurs in the middle atmosphere due to four exothermic reactions involving members of the odd-hydrogen family is calculated. The following reactions are considered: O + OH yields O2 + H; H + O2 + M yields HO2 + M; H + O3 yields OH + O2; and O + HO2 yields OH + O2. It is shown that the heating rates due to these reactions rival the oxygen-related heating rates conventionally considered in middle-atmosphere models. The conversion of chemical potential energy into molecular translational energy (heat) by these odd-hydrogen reactions is shown to be a significant energy source in the middle atmosphere that has not been previously considered.

Mlynczak, Martin G.; Solomon, Susan

1991-01-01

456

Analysis of parameter effects on transport phenomena in conjunction with chemical reactions in ducts relevant for methane reformers  

Microsoft Academic Search

Various transport phenomena in conjunction with chemical reactions are strongly affected by reformer configurations and properties of involved porous catalyst layers. The considered composite duct is relevant for a methane steam reformer and consists of a porous layer for the catalytic chemical reactions, the fuel gas flow duct and solid plate. In this paper, a fully three-dimensional calculation method is

Jinliang Yuan; XinRong Lv; Bengt Sundén; Dantin Yue

2007-01-01

457

CO2 in 1-butyl-3-methylimidazolium acetate. 2. NMR investigation of chemical reactions.  

PubMed

The solvation of CO(2) in 1-butyl-3-methylimidazolium acetate (Bmim Ac) has been investigated by (1)H, (13)C, and (15)N NMR spectroscopy at low CO(2) molar fraction (mf) (x(CO(2)) ca. 0.27) corresponding to the reactive regime described in part 1 of this study. It is shown that a carboxylation reaction occurs between CO(2) and Bmim Ac, leading to the formation of a non-negligible amount (~16%) of 1-butyl-3-methylimidazolium-2-carboxylate. It is also found that acetic acid molecules are produced during this reaction and tend to form with elapsed time stable cyclic dimers existing in pure acid. A further series of experiments has been dedicated to characterize the influence of water traces on the carboxylation reaction. It is found that water, even at high ratio (0.15 mf), does not hamper the formation of the carboxylate species but lead to the formation of byproduct involving CO(2). The evolution with temperature of the resonance lines associated with the products of the reactions confirms that they have a different origin. The main byproduct has been assigned to bicarbonate. All these results confirm the existence of a reactive regime in the CO(2)-Bmim Ac system but different from that reported in the literature on the formation of a reversible molecular complex possibly accompanied by a minor chemical reaction. Finally, the reactive scheme interpreting the carboxylation reaction and the formation of acetic acid proposed in the literature is discussed. We found that the triggering of the carboxylation reaction is necessarily connected with the introduction of carbon dioxide in the IL. We argue that a more refined scheme is still needed to understand in details the different steps of the chemical reaction in the dense phase. PMID:22533570

Besnard, Marcel; Cabaço, M Isabel; Vaca Chávez, Fabián; Pinaud, Noël; Sebastião, Pedro J; Coutinho, João A P; Mascetti, Joëlle; Danten, Yann

2012-05-24

458

Chemical Reactions  

NSDL National Science Digital Library

Though titled for kids the information presented at this site is thorough and supported with hot links to related concepts as well as illustrations. Click on the blue Next Stop on the Tour button to continue learning.

2009-01-01

459

Chemical Reactions  

NSDL National Science Digital Library

SciPacks are 10 hour online learning experiences teachers can use to enhance their understanding of a particular scientific concept. Teachers access topics "on demand" from the Internet. Topics are based on science literacy goals in the national standards

National Science Teachers Association (NSTA)

2009-07-10

460

Gas chromatography plasma-assisted reaction chemical ionization mass spectrometry for quantitative detection of bromine in organic compounds.  

PubMed

We have recently introduced plasma-assisted reaction chemical ionization mass spectrometry (PARCI-MS) for elemental analysis of halogens in organic compounds. Here, we utilize gas chromatography (GC) coupled to PARCI-MS to investigate the mechanism of Br(-) ion generation from organobromines and to evaluate analytical performance of PARCI for organobromine analysis. Bromine atoms in compounds eluting from GC are converted to HBr in a low-pressure microwave induced helium plasma with trace amounts of hydrogen added as a reaction gas. Ionization is achieved by introducing nitrogen into the afterglow region of the plasma, liberating electrons via penning ionization and leading to formation of negative ions. We demonstrate that N2 largely affects the ionization process, whereas H2 affects both the ionization process and in-plasma reactions. Our investigations also suggest that dissociative electron capture is the main ionization route for formation of Br(-) ions. Importantly, GC-PARCI-MS shows a uniform response factor for bromine across brominated compounds of drastically different chemical structures, confirming PARCI's ability to quantify organobromines in the absence of compound-specific standards. Over 3 orders of magnitude linear dynamic range is demonstrated for bromine quantification. We report a detection limit of 29 fg of bromine on-column, ~4-fold better than inductively coupled plasma-MS. PMID:25003497

Lin, Ninghang; Wang, Haopeng; Kahen, Kaveh; Badiei, Hamid; Jorabchi, Kaveh

2014-08-01

461

A coupled mechanical and chemical damage model for concrete affected by alkali–silica reaction  

SciTech Connect

To model the complex degradation phenomena occurring in concrete affected by alkali–silica reaction (ASR), we formulate a poro-mechanical model with two isotropic internal variables: the chemical and the mechanical damage. The chemical damage, related to the evolution of the reaction, is caused by the pressure generated by the expanding ASR gel on the solid concrete skeleton. The mechanical damage describes the strength and stiffness degradation induced by the external loads. As suggested by experimental results, degradation due to ASR is considered to be localized around reactive sites. The effect of the degree of saturation and of the temperature on the reaction development is also modeled. The chemical damage evolution is calibrated using the value of the gel pressure estimated by applying the electrical diffuse double-layer theory to experimental values of the surface charge density in ASR gel specimens reported in the literature. The chemo-damage model is first validated by simulating expansion tests on reactive specimens and beams; the coupled chemo-mechanical damage model is then employed to simulate compression and flexure tests results also taken from the literature. -- Highlights: •Concrete degradation due to ASR in variable environmental conditions is modeled. •Two isotropic internal variables – chemical and mechanical damage – are introduced. •The value of the swelling pressure is estimated by the diffuse double layer theory. •A simplified scheme is proposed to relate macro- and microscopic properties. •The chemo-mechanical damage model is validated by simulating tests in literature.

Pignatelli, Rossella, E-mail: rossellapignatelli@gmail.com [Department of Civil and Environmental Engineering, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano (Italy) [Department of Civil and Environmental Engineering, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano (Italy); Lombardi Ingegneria S.r.l., Via Giotto 36, 20145 Milano (Italy); Comi, Claudia, E-mail: comi@stru.polimi.it [Department of Civil and Environmental Engineering, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano (Italy)] [Department of Civil and Environmental Engineering, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano (Italy); Monteiro, Paulo J.M., E-mail: monteiro@ce.berkeley.edu [Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720 (United States)

2013-11-15

462

Chemical dynamics of vibrationally excited molecules: Controlling reactions in gases and on surfaces  

PubMed Central

Experimental studies of the chemical reaction dynamics of vibrationally excited molecules reveal the ability of different vibrations to control the course of a reaction. This Perspective describes those studies for the prototypical reaction of vibrationally excited methane and its isotopologues in gases and on surfaces and looks to the prospects of similar studies in liquids. The influences of vibrational excitation on the CH bond cleavage in a single collision reaction with Cl and in dissociative adsorption on a Ni surface bear some striking similarities. Both reactions are bond-selective processes in which the initial preparation of a molecular eigenstate containing a large component of CH stretching results in preferential cleavage of that bond. It is possible to cleave either the CH bond or CD bond in the reaction of Cl with CH3D, CH2D2, or CHD3 and, similarly, to use initial excitation of the CH stretch to promote dissociation of CHD3 to CD3 and H on a Ni surface. Different vibrational modes, such as the symmetric and antisymmetric stretches in CH3D or CH4, lead to very different reactivities, and molecules with the symmetric stretching vibration excited can be as much as 10 times more reactive than ones with the antisymmetric stretch excited. The origin of this behavior lies in the change in the vibrational motion induced by the interaction with the atomic reaction partner or the surface. PMID:18765816

Crim, F. Fleming

2008-01-01

463

Mineral growth in metamorphic rocks: relationships between chemical patterns, mineral microstructure and reaction kinetics  

NASA Astrophysics Data System (ADS)

Mineral growth in metamorphic rocks is governed by kinetic processes, which are strongly related to geometrical and physical properties of the chemical pathways involved. Information on the nature of chemical pathways and potential effects of deformation on kinetic processes is stored in chemical patterns and mineral microstruture. By using high-resolution analytical techniques (i.e. EBSD, FE-EMPA and TEM/FIB) and numerical simulations we explored natural and experimental samples in order to reveal the kinetics and the microstrutural evolution of both transport controlled mineral reactions and exchange reactions. We found reaction rims between reactant minerals, which are composed of several grains. These grains are subdivided into subgrains perpendicular to the reaction fronts. The subgrains often show a systematic lateral orienation change, which is possibly caused by grain-scale plastic deformation in the form of creep polygonization. Such an arrangment of grain boundaries allows for short-circuit diffusion and amplifies the necessary mass transfer across reaction rims. Information about the contribution of grain boundary diffusion to bulk material flow in Grt is stored during the formation of asymmetric growth zonings and during micron- scale compositional variations along grain and phase boundaries. Our evaluation of these compositional patterns by diffusion modelling allows for the derivation of Digb/Divol ratios and rim growth rates. Information about phase boundary material properties is also stored in the zoning of Grt porphyroblasts, which formed during the retrograde Fe-Mg exchange between Grt and biotite. If temperature falls below a limiting value during cooling, non-equilibrated rim compositions start to develop along Grt-Qtz phase boundaries due to insufficient chemical transport rates. As the extent of non-equilibrated rim compositions reflects the transport capacity of the present interganular medium, our simulations by diffusion modelling of observed compositional patterns formed during cooling at Grt-Bt-Qtz triple junctions allows for the extraction of boundary material properties.

Keller, L. M.

2008-12-01

464

A Non-Orthogonal Block-Localized Effective Hamiltonian Approach for Chemical and Enzymatic Reactions  

PubMed Central

The effective Hamiltonian-molecular orbital and valence bond (EH-MOVB) method based on non-orthogonal block-localized fragment orbitals has been implemented into the program CHARMM for molecular dynamics simulations of chemical and enzymatic reactions, making use of semiempirical quantum mechanical models. Building upon ab initio MOVB theory, we make use of two parameters in the EH-MOVB method to fit the barrier height and the relative energy between the reactant and product state for a given chemical reaction to be in agreement with experiment or high-level ab initio or density functional results. Consequently, the EH-MOVB method provides a highly accurate and computationally efficient QM/MM model for dynamics simulation of chemical reactions in solution. The EH-MOVB method is illustrated by examination of the potential energy surface of the hydride transfer reaction from trimethylamine to a flavin cofactor model in the gas phase. In the present study, we employed the semiempirical AM1 model, which yields a reaction barrier that is more than 5 kcal/mol too high. We use a parameter calibration procedure for the EH-MOVB method similar to that employed to adjust the results of semiempirical and empirical models. Thus, the relative energy of these two diabatic states can be shifted to reproduce the experimental energy of reaction, and the barrier height is optimized to reproduce the desired (accurate) value by adding a constant to the off-diagonal matrix element. The present EH-MOVB method offers a viable approach to characterizing solvent and protein-reorganization effects in the realm of combined QM/MM simulations. PMID:20694172

Cembran, Alessandro; Payaka, Apirak; Lin, Yen-lin; Xie, Wangshen; Mo, Yirong; Song, Lingchun; Gao, Jiali

2010-01-01

465

Chemical reactions of organic molecules adsorbed on ice 1. Chlorine addition to propene  

SciTech Connect

Chemical reactions that are promoted or catalyzed by the surface or near surface regions of ice particles in type II polar stratospheric clouds (PSCs) have been shown to play a role in the annual sequence of events that ultimately opens the Antarctic ozone hole. A new ice-catalyzed chemical reaction is reported, namely the addition of Cl{sub 2} to the C--C double bond of propene (C{sub 3}H{sub 6}) to form 1,2-dichloropropane. The reaction, which was investigated with temperature-programmed desorption mass spectrometry (TPD), was carried out on ultrathin (10--100 monolayer thick) films of ice deposited on single-crystal metal substrates under ultrahigh vacuum. The Cl{sub 2}-addition product was identified as 1,2-dichloropropane on the basis of its fragmentation pattern in the mass spectrometer. 1,2-Dichloropropane formation occurs below 150 K, and no chlorohydrin (e.g., 2-chloro-1-propanol) evolution is ever observed. The reaction of coadsorbed propene and chlorine is very different from what occurs in aqueous solutions, where chlorohydrin formation occurs readily. Possible mechanisms of the ice-surface-catalyzed reaction are discussed.

Graham, J.D.; Roberts, J.T.

2000-02-10

466

The role of gas-phase reactions in modeling of the forced-flow chemical vapor infiltration process  

Microsoft Academic Search

An analytical model is presented, which includes the effects of both gas-phase and surface reactions, and the pressure changes due to the chemical reactions in the forced-flow chemical vapor infiltration (FCVI) process. For the FCVI process controlled by the gas-phase reactions, improvements of the process by using the forced-flow are limited. However, for the FCVI process controlled by the surface

Ching Yi Tsai; J. N. Reddy; Seshu B. Desu; Chien C. Chiu

1993-01-01

467

Influence of Alumina Reaction Tube Impurities on the Oxidation of Chemically-Vapor-Deposited Silicon Carbide  

NASA Technical Reports Server (NTRS)

Pure coupons of chemically vapor deposited (CVD) SiC were oxidized for 100 h in dry flowing oxygen at 1300 C. The oxidation kinetics were monitored using thermogravimetry (TGA). The experiments were first performed using high-purity alumina reaction tubes. The experiments were then repeated using fused quartz reaction tubes. Differences in oxidation kinetics, scale composition, and scale morphology were observed. These differences were attributed to impurities in the alumina tubes. Investigators interested in high-temperature oxidation of silica formers should be aware that high-purity alumina can have significant effects on experiment results.

Opila, Elizabeth

1995-01-01